Method and base station for transmitting reference signals, and method and user equipment for receiving reference signals

ABSTRACT

A method and apparatus for multiplexing reference signals in a predetermined number of Code Division Multiplexing (CDM) groups to balance power across Orthogonal Frequency Division Multiplexing (OFDM) symbols are disclosed. In a wireless communication system, orthogonal sequences used for spreading the reference signals are allocated such that the order of orthogonal sequences allocated to a subcarrier of one CDM group has a predetermined offset with respect to the order of orthogonal sequences allocated to a subcarrier of another CDM group, adjacent to the subcarrier of the one CDM group.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. parent application Ser. No.13/049,847, filed on Mar. 16, 2011, now U.S. Pat. No. 9,019,904, whichclaims the benefit of earlier filing date and right of priority toKorean Application No. 10-2011-0011806, filed on Feb. 10, 2011, and alsoclaims the benefit of U.S. Provisional Application Nos. 61/376,174,filed on Aug. 23, 2010, 61/331,314, filed on May 4, 2010, 61/324,234,filed on Apr. 14, 2010, 61/315,398, filed on Mar. 19, 2010, 61/315,023,filed on Mar. 18, 2010, and 61/314,544, filed on Mar. 16, 2010, thecontents of which are all hereby incorporated by reference herein intheir entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for transmitting ReferenceSignals (RSs) for data demodulation and a method and apparatus forreceiving RSs for data demodulation.

2. Discussion of the Related Art

Multiple Input Multiple Output (MIMO) has recently attracted interest tomaximize the performance and communication capacity of a wirelesscommunication system. Compared to conventional use of a singleTransmission (Tx) antenna and a single Reception (Rx) antenna, MIMOadopts a plurality of Tx antennas and a plurality of Rx antennas tothereby increase the transmission and reception efficiency of data. AMIMO system is called a multiple antenna system. MIMO is an applicationof putting data segments received from a plurality of antennas into awhole message, without depending on a single antenna path to receive thewhole message. Consequently, MIMO can increase data transmission ratewithin a given area or extend system coverage at a given datatransmission rate.

MIMO schemes are classified into transmit diversity, spatialmultiplexing, and beamforming. Transmit diversity increases transmissionreliability by transmitting the same data through multiple Tx antennas.In spatial multiplexing, multiple Tx antennas simultaneously transmitdifferent data and thus high-speed data can be transmitted withoutincreasing a system bandwidth. Beamforming is used to increase theSignal-to-Interference plus Noise Ratio (SINR) of a signal by weightingmultiple antennas according to channel states. Weights may be expressedas a weight vector or a weight matrix, called a precoding vector or aprecoding matrix.

Spatial multiplexing is further divided into spatial multiplexing for asingle user (or Single User MIMO (SU-MIMO)) and spatial multiplexing formultiple users (or Multi-User MIMO (MU-MIMO)).

A Base Station (BS) may transmit a plurality of layers for one or moreusers. For this purpose, the BS multiplexes the layers into apredetermined time/frequency area and transmits the multiplexed layersto one or more User Equipments (UEs). In general, maximum transmissionpower available for downlink transmission of the BS is determined by thesupported frequency bandwidth, data throughput, and power efficiency ofthe BS. Because the total transmission power available to the BS islimited to a predetermined value, the BS needs to efficiently allocatetransmission power to each subcarrier in an Orthogonal FrequencyDivision Multiplexing (OFDM) symbol interval.

To demodulate data allocated to a predetermined time/frequency area, aUE estimates the configuration of physical antennas used for the datatransmission and channel quality using an RS received from the BS, thatis, the UE performs channel estimation using the received RS. Channelestimation and an RS will be described in brief. To detect asynchronization signal, a receiver should have information about a radiochannel (e.g. the attenuation, phase shift, time delay, etc. of theradio channel). Channel estimation is the process of estimating themagnitude and reference phase of a carrier. A wireless channelenvironment is characterized by irregular variations of channel stateover time, called fading. The amplitude and phase of the fading channelare estimated through channel estimation. That is, channel estimationrefers to estimating the frequency response of a radio interface orradio channel. For channel estimation, a reference value is estimatedusing some RSs of a BS by a two-dimensional channel estimator. An RS isdefined as a symbol with high power without carrying actual data inorder to help carrier phase synchronization and BS informationacquisition. A transmitter and a receiver can perform channel estimationusing such RSs. The receiver can recover data received from thetransmitter based on the result of RS-based channel estimation.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a method and basestation for transmitting reference signals and a method and userequipment for receiving reference signals that substantially obviate oneor more problems due to limitations and disadvantages of the relatedart.

An object of the present invention is to appropriately configureReference Signals (RSs) for demodulation of a signal transmitted by atransmitter so that a receiver may accurately demodulate the signalusing the RSs.

Another object of the present invention is to configure RSs in such amanner that appropriate transmission power may be allocated to RSs fordemodulation within a total transmission power available to a BS toallow a receiver to receive the RSs with high accuracy.

A further object of the present invention is to uniformly distributepower to Orthogonal Frequency Division Multiplexing (ODM) symbols sothat a BS may efficiently utilize its available power.

It is to be understood that technical objects to be achieved by thepresent invention are not limited to the aforementioned technicalobjects and other technical objects which are not mentioned will beapparent from the following description to the person with an ordinaryskill in the art to which the present invention pertains.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein,orthogonal sequences used for spreading RSs are allocated such that theorder of orthogonal sequences allocated to a subcarrier of one CodeDivision Multiplexing (CDM) group has a predetermined offset withrespect to the order of orthogonal sequences allocated to a subcarrierof another CDM group, adjacent to the subcarrier of the one CDM group.

After phase offsets are applied to RSs of layers according to thelayers, the RSs are multiplexed in predetermined radio resources.

In one aspect of the present invention, a method for transmitting aplurality of RSs to a UE at a BS in a wireless communication systemincludes spreading the plurality of RSs with spreading orthogonalsequences, and transmitting the plurality of RSs on at least one of afirst CDM group and a second CDM group. RSs being transmitted on thefirst CDM group among the plurality of RSs are spread with one oforthogonal spreading sequences listed in a first table and transmittedon a subcarrier of the first CDM group, and RSs being transmitted on thesecond CDM group among the plurality of RSs are spread with one oforthogonal spreading sequences listed in a second table and transmittedon a subcarrier of the second CDM group.

In another aspect of the present invention, a method for receiving aplurality of RSs from a BS at a UE in a wireless communication systemincludes receiving the plurality of RSs on at least one of a first CDMgroup and a second CDM group from the BS, and detecting a first RSdestined for the UE from among the plurality of RSs, using a firstspreading orthogonal sequence used for spreading the first RS by the BS.If the first RS is received on the first CDM group, the first spreadingorthogonal sequence is one of orthogonal spreading sequences listed in afirst table, and if the first RS is received in a second CDM group, thefirst spreading orthogonal sequence is one of orthogonal spreadingsequences listed in a second table.

In another aspect of the present invention, a BS for transmitting aplurality of RSs to a UE in a wireless communication system includes atransmitter, and a processor for controlling the transmitter. Theprocessor controls the transmitter to spread the plurality of RSs withspreading orthogonal sequences and transmit the plurality of RSs on atleast one of a first CDM group and a second CDM group. The processorcontrols the transmitter to spread RSs to be transmitted on the firstCDM group among the plurality of RSs with one of orthogonal spreadingsequences listed in the a first table and transmit the spread RSs on asubcarrier of the first CDM group, and the processor controls thetransmitter to spread RSs to be transmitted on the second CDM groupamong the plurality of RSs with one of orthogonal spreading sequenceslisted in a second table and transmit the spread RSs on a subcarrier ofthe second CDM group.

In a further aspect of the present invention, a UE for receiving aplurality of RSs from a BS in a wireless communication system includes areceiver, and a processor for controlling the receiver. The processorcontrols the receiver to receive the plurality of RSs on at least one ofa first CDM group and a second CDM group from the BS, and controls thereceiver to detect a first RS destined for the UE from among theplurality of RSs, using a first spreading orthogonal sequence used forspreading the first RS by the BS. If the first RS is received in thefirst CDM group, the first spreading orthogonal sequence is one oforthogonal spreading sequences listed in a first table, and if the firstRS is received in the second CDM group, the first spreading orthogonalsequence is one of orthogonal spreading sequences listed in a secondtable.

In each aspect of the present invention, the first table may be

Orthogonal sequence [w_(p)(0) w_(p)(1) w_(p)(2) w_(p)(3)] [+1 +1 +1 +1][+1 −1 +1 −1] [+1 +1 −1 −1] [+1 −1 −1 +1]and the second table may be

Orthogonal sequence [w_(p)(0) w_(p)(1) w_(p)(2) w_(p)(3)] [+1 +1 +1 +1][+1 −1 +1 −1] [−1 −1 +1 +1] [−1 +1 +1 −1]

In each aspect of the present invention, the plurality of RSs may bespread according to a third table and transmitted on at least one of thefirst and second CDM groups. The third table may be

Orthogonal sequence RS [w_(p)(0) w_(p)(1) w_(p)(2) w_(p)(3)] CDM group 0[+1 +1 +1 +1] 1 1 [+1 −1 +1 −1] 1 2 [+1 +1 +1 +1] 2 3 [+1 −1 +1 −1] 2 4[+1 +1 −1 −1] 1 5 [−1 −1 +1 +1] 2 6 [+1 −1 −1 +1] 1 7 [−1 +1 +1 −1] 2where RS 0 to RS 7 one-to-one correspond to layer 0 to layer 7.

In each aspect of the present invention, the plurality of RSs may bemultiplexed in at least one of the first and second CDM groups, usingmultiplexing orthogonal sequences a, b, c and d defined as

$\begin{matrix}{W_{4} = \begin{pmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {+ 1} & {- 1} & {- 1} \\{+ 1} & {- 1} & {- 1} & {+ 1}\end{pmatrix}} \\{= \begin{pmatrix}a & b & c & d\end{pmatrix}}\end{matrix}$RS 0, RS 1, RS 4 and RS 6 may be multiplexed in the first CDM group by(RS0 RS1 RS4 RS6)×(a b c d) and RS 2, RS 3, RS 5 and RS 7 may bemultiplexed in the second CDM group by (RS2 RS3 RS5 RS7)×(c d a b).

In each aspect of the present invention, the plurality of RSs may bemultiplexed in two adjacent subcarriers of the first and second CDMgroups in a symbol, using one of multiplexing orthogonal sequence pairs(a, c) and (b, d).

In each aspect of the present invention, RS p for layer p, r(m) may beallocated to the first or second CDM group according to the followingFormula.

$a_{k,l}^{p} = {{{\overset{\_}{w}}_{p}( l^{\prime} )} \cdot {r( {{3 \cdot l^{\prime} \cdot N_{RB}^{\max,{DL}}} + {3 \cdot n_{PRB}} + m^{\prime}} )}}$where ${{\overset{\_}{w}}_{p}(i)} = \{ {{\begin{matrix}{w_{p}(i)} & {{( {m^{\prime} + n_{PRB}} ){mod}\; 2} = 0} \\{w_{p}( {3 - i} )} & {{( {m^{\prime} + n_{PRB}} ){mod}\; 2} = 1}\end{matrix}k} = {{{5 \cdot m^{\prime}} + {N_{sc}^{RB} \cdot n_{PRB}} + {k^{\prime}k^{\prime}}} = \{ {{\begin{matrix}1 & {p \in \{ {0,1,4,6} \}} \\0 & {p \in \{ {2,3,5,7} \}}\end{matrix}l^{\prime}} = \{ {{\begin{matrix}{{l^{\prime}{mod}\; 2} + 2} & {{{if}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 3},4,{{or}\mspace{14mu} 8}} \\{{l^{\prime}{mod}\; 2} + 2 + {3 \cdot \lfloor {l^{\prime}/2} \rfloor}} & {{{if}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 1},2,6,{{or}\mspace{14mu} 7}} \\{{l^{\prime}{mod}\; 2} + 5} & {{if}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}}\end{matrix}l^{\prime}} = \{ {{{\begin{matrix}{0,1,2,3} & {{{{if}\mspace{14mu} n_{s}{mod}\; 2} = {0\mspace{14mu}{and}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 1}},2,6,{{or}\mspace{14mu} 7}} \\{0,1} & {{{{if}\mspace{14mu} n_{s}{mod}\; 2} = {0\mspace{14mu}{and}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 1}},2,6,{{or}\mspace{14mu} 7}} \\{2,3} & {{{{if}\mspace{14mu} n_{s}{mod}\; 2} = {1\mspace{14mu}{and}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 1}},2,6,{{or}\mspace{14mu} 7}}\end{matrix}m^{\prime}} = 0},1,2} } } }} $where n_(PRB) is the index of a Physical Resource Block (PRB), N^(RB)_(sc) is the number of subcarriers in an RB, N^(max,DL) _(RB) is themaximum number of RBs in a downlink slot, p is the index of a layer, kand l are a subcarrier index and an OFDM symbol index in a subframe, m′is a counter of RS subcarriers carrying RSs in an RB, and l′ is acounter of RS OFDM symbols including RBs in a subframe.

In each aspect of the present invention, the plurality of RSs may bemultiplexed in a subframe in the pattern of FIG. 31( b) and transmittedto the UE by the BS.

The aforementioned technical solutions are only a part of theembodiments of the present invention, and various modifications to whichtechnical features of the present invention are applied could beunderstood by the person with ordinary skill in the art to which thepresent invention pertains, based on the following detailed descriptionof the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a block diagram of a User Equipment (UE) and a Base Station(BS) for implementing the present invention.

FIG. 2 is a block diagram of an exemplary transmitter in each of the UEand the BS.

FIG. 3 illustrates an exemplary structure of a radio frame in a wirelesscommunication system.

FIG. 4 illustrates an exemplary Downlink/Uplink (DL/UL) slot structurein the wireless communication system.

FIG. 5 illustrates an exemplary DL subframe in the wirelesscommunication system.

FIG. 6 is a conceptual view of Dedicated Reference Signal (DRS)transmission.

FIG. 7 illustrates exemplary DRS patterns in a Long Term Evolution (LTE)system.

FIG. 8 illustrates an exemplary DRS pattern in an LTE-Advanced (LTE-A)system.

FIG. 9 illustrates exemplary patterns of multiplexing DRSs for twolayers in a subframe with a normal Cyclic Prefix (CP), using OrthogonalCover Codes (OCCs) of length 2.

FIG. 10 illustrates an exemplary transmission of DRSs for four layers intwo Code Division Multiplexing (CDM) groups.

FIG. 11 illustrates a method for multiplexing four DRSs in two CDMgroups.

FIG. 12 illustrates exemplary multiplexing of two DRSs in one CDM group.

FIG. 13 illustrates FIG. 13 illustrates an embodiment of the presentinvention for uniformly distributing transmission power acrossOrthogonal Frequency Division Multiplexing (OFDM) symbols in rank-2transmission.

FIGS. 14 and 15 illustrate exemplary power allocations to DRS ResourceElements (REs) and data REs in rank-2 transmission.

FIG. 16 illustrates an example of allocating DRSs for layerscorresponding to antenna ports 11 to 14 in two CDM groups.

FIG. 17 illustrates a method for multiplexing eight DRSs in two CDMgroups.

FIGS. 18 to 22 illustrate multiplexing of DRSs in one CDM group usingOCCs of length 4 according to embodiments of the present invention.

FIGS. 23 to 30 illustrate multiplexing of DRSs in two CDM groups usingOCCs of length 4 according to embodiments of the present invention.

FIG. 31 illustrates OCC allocation so that there is a predetermined OCCoffset between two CDM groups according to embodiments of the presentinvention.

FIGS. 32 to 38 are views referred to for describing advantages ofallocating OCCs so that there is a predetermined OCC offset between twoCDM groups according to embodiments of the present invention.

FIG. 39 illustrates exemplary phase offsets for DRS subcarriers ofrespective DRS ports.

FIGS. 40, 41 and 42 are views referred to for describing advantages ofapplying phase offsets according to DRS subcarriers for each layeraccording to embodiments of the present invention.

FIG. 43 is a view referred to for describing advantages achieved whenOCCs are allocated so that there is a predetermined OCC offset betweentwo CDM groups and phase offsets are applied according to DRSsubcarriers for each layer according to embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. It is to beunderstood that the detailed description, which will be disclosed alongwith the accompanying drawings, is intended to describe the exemplaryembodiments of the present invention, and is not intended to describe aunique embodiment with which the present invention can be carried out.The following detailed description includes detailed matters to providefull understanding of the present invention. However, it will beapparent to those skilled in the art that the present invention can becarried out without the detailed matters.

Techniques, apparatuses and systems described herein can be used invarious wireless access technologies such as Code Division MultipleAccess (CDMA), Frequency Division Multiple Access (FDMA), Time DivisionMultiple Access (TDMA), Orthogonal Frequency Division Multiple Access(OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA),etc. CDMA may be implemented with a radio technology such as UniversalTerrestrial Radio Access (UTRA) or CDMA2000. TDMA may be implementedwith a radio technology such as Global System for Mobile communications(GSM)/General Packet Radio Service (GPRS)/Enhanced Data Rates for GSMEvolution (EDGE). OFDMA may be implemented with a radio technology suchas Institute of Electrical and Electronics Engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, Evolved-UTRA (E-UTRA) etc.UTRA is a part of Universal Mobile Telecommunication System (UMTS). 3rdGeneration Partnership Project (3GPP) Long Term Evolution (LTE) is apart of Evolved-UMTS (E-UMTS) using E-UTRA. 3GPP LTE employs OFDMA indownlink and employs SC-FDMA in uplink. LTE-Advanced (LTE-A) is anevolution of 3GPP LTE. For clarity, this application focuses on 3GPPLTE/LTE-A. However, technical features of the present invention are notlimited thereto. For example, although the following description will begiven in the context of a 3GPP LTE/LTE-A system being used as a mobilecommunication system, the following description is also applicable toother mobile communication systems except unique features of the 3GPPLTE/LTE-A system.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, wherever possible, thesame reference numbers will be used throughout the drawings and thespecification to refer to the same or like parts.

In the present invention, a User Equipment (UE) denotes a mobile orfixed type user terminal. Examples of the UE include various equipmentsthat transmit and receive user data and/or various kinds of controlinformation to and from a Base Station (BS). The UE may be referred toas, a Terminal Equipment (TE), a Mobile Station (MS), a Mobile Terminal(MT), a User Terminal (UT), a Subscriber Station (SS), a wirelessdevice, a Personal Digital Assistant (PDA), a wireless modem, or ahandheld device. Also, in the present invention, a BS means a fixedstation that performs communication with a UE and/or another BS, andexchanges various kinds of data and control information with the UE andanother BS. The BS may be referred to as another terminology such as anevolved-Node B (eNB), a Base Transceiver System (BTS), and an AccessPoint (AP).

Hereinafter, a Physical Downlink Control CHannel (PDCCH)/PhysicalControl Format Indicator CHannel (PCFICH)/Physical Hybrid ARQ IndicatorCHannel (PHICH)/Physical Downlink Shared CHannel (PDSCH)/DedicatedReference Signal (DRS)/Common Reference Signal (CRS)/DeModulationReference Signal (DMRS)/Channel State Information-Reference Signal(CSI-RS) Resource Element (RE) represents an RE assigned to or availablefor PDCCH/PCFICH/PHICH/PDSCH/DRS/CRS/DMRS/CSI-RS. Especially, an REcarrying an RS is referred to as an RS RE and an RE carrying controlinformation or data is referred to as a data RE.

Hereinafter, a symbol/carrier/subcarrier to which a DRS/CRS/DMRS/CSI-RSis allocated will be referred to as a DRS/CRS/DMRS/CSI-RSsymbol/carrier/subcarrier. For example, a symbol carrying a CSI-RS isreferred to as a CSI-RS symbol and a subcarrier carrying a CSI-RS isreferred to as a CSI-RS subcarrier. In addition, a symbol carrying userdata (e.g. PDSCH data, PDCCH data, etc.) is referred to as a data symboland a subcarrier carrying user data is referred to as a data subcarrier.

Meanwhile, in the present invention, if a specific signal is allocatedto a frame, subframe, slot, symbol, carrier, or subcarrier, it meansthat the specific signal is transmitted through the correspondingcarrier or subcarrier during a period/timing of the corresponding frame,subframe, slot or symbol.

According to the present invention, a rank or transmission rank is thenumber of layers multiplexed/allocated to an OFDM symbol or data RE.

Hereinafter, if a specific signal within a frame, subframe, slot,symbol, carrier or subcarrier is not transmitted, it will be expressedthat transmission of the specific signal has been dropped, muted, nulledor blanked in the frame, the subframe, the slot, the symbol, the carrieror the subcarrier. For example, if a transmitter transmits a specificsignal with zero transmission power on a predetermined RE, it may besaid that the transmitter has dropped transmission of the specificsignal, has muted or blanked the predetermined RE, or transmits a nullsignal on the RE.

FIG. 1 is a block diagram of a UE and a BS for implementing the presentinvention.

The UE serves as a transmitter on the uplink and as a receiver on thedownlink. In contrast, the BS may serve as a receiver on the uplink andas a transmitter on the downlink.

The UE and the BS include antennas 500 a and 500 b for receivinginformation, data, signals, and/or messages, transmitters 100 a and 100b for transmitting messages by controlling the antennas 500 a and 500 b,receivers 300 a and 300 b for receiving messages by controlling theantennas 500 a and 500 b, and memories 200 a and 200 b for storinginformation associated with communication in the wireless communicationsystem. The UE and the BS further include processors 400 a and 400 b,respectively, which are operative coupled to the components of the UEand the BS, such as the transmitters 100 a and 100 b, the receivers 300a and 300 b, and the memories 200 a and 200 b, and adapted to performthe present invention by controlling the components of the UE and theBS. The transmitter 100 a, the memory 200 a, the receiver 300 a, and theprocessor 400 a in the UE may be configured as independent components onseparate chips or their separate chips may be incorporated into a singlechip. Likewise, the transmitter 100 b, the memory 200 b, the receiver300 b, and the processor 400 b in the BS may be configured asindependent components on separate chips or their separate chips may beincorporated into a single chip. The transmitter and the receiver may beconfigured as a single transceiver or a Radio Frequency (RF) module inthe UE or the BS.

The antennas 500 a and 500 b transmit signals generated from thetransmitters 100 a and 100 b to the outside, or transfer radio signalsreceived from the outside to the receivers 300 a and 300 b. The antennas500 a and 500 b may be referred as antenna ports. Each antenna port maycorrespond to one physical antenna or may be configured into acombination of more than one physical antenna. If the transmitters 100 aand 100 b and/or the receivers 300 a and 300 b support a Multiple InputMultiple Output (MIMO) function using a plurality of antennas, each ofthem may be connected to two or more antennas.

The processors 400 a and 400 b generally provide overall control to themodules of the UE and the BS. Especially, the processors 400 a and 400 bmay carry out a control function for performing the present invention, aMedium Access Control (MAC) frame variable control function based onservice characteristics and a propagation environment, a power savingmode function for controlling idle-mode operations, a handover function,and an authentication and encryption function. The processors 400 a and400 b may also be referred to as controllers, microcontrollers,microprocessors, microcomputers, etc. The processors 400 a and 400 b maybe configured in hardware, firmware, software, or their combination. Ina hardware configuration, the processors 400 a and 400 b may be providedwith one or more Application Specific Integrated Circuits (ASICs),Digital Signal Processors (DSPs), Digital Signal Processing Devices(DSPDs), Programmable Logic Devices (PLDs), and/or Field ProgrammableGate Arrays (FPGAs), for implementing the present invention. In afirmware or software configuration, firmware or software may beconfigured to include a module, a procedure, a function, etc. forperforming functions or operations of the present invention. Thisfirmware or software may be provided in the processors 400 a and 400 b,or may be stored in the memories 200 a and 200 b and driven by theprocessors 400 a and 400 b.

The transmitters 100 a and 100 b perform predetermined coding andmodulation for signals and/or data, which are scheduled by schedulersconnected to the processors 400 a and 400 b and transmitted to theoutside, and then transfer the modulated signals and/or data to theantennas 500 a and 500 b. For example, the transmitters 100 a and 100 bconvert a transmission data stream to K layers by demultiplexing,channel coding, modulation, etc. The K layers are transmitted throughthe antennas 500 a and 500 b after being processed in transmissionprocessors of the transmitters 100 a and 100 b. The transmitters 100 aand 100 b and the receivers 300 a and 300 b of the UE and the BS may beconfigured in different manners depending on the procedures ofprocessing transmitted signals and received signals.

The memories 200 a and 200 b may store programs required for signalprocessing and controlling of the processors 400 a and 400 b andtemporarily store input and output information. The memories 200 a and200 b may be used as buffers. Each of the memories 200 a and 200 b maybe implemented into a flash memory-type storage medium, a hard disc-typestorage medium, a multimedia card micro-type storage medium, a card-typememory (e.g. a Secure Digital (SD) or eXtreme Digital (XS) memory), aRandom Access Memory (RAM), a Read-Only Memory (ROM), an ElectricallyErasable Programmable Read-Only Memory (EEPROM), a ProgrammableRead-Only Memory (PROM), a magnetic memory, a magnetic disc, or anoptical disk.

FIG. 2 is a block diagram of an exemplary transmitter in each of the UEand the BS. Operations of the transmitters 100 a and 100 b will bedescribed below in more detail with reference to FIG. 2.

Referring to FIG. 2, each of the transmitters 100 a and 100 b includescramblers 301, modulation mappers 302, a layer mapper 303, a precoder304, RE mappers 305, Orthogonal Frequency Division Multiplexing/SingleCarrier Frequency Division Multiplexing (OFDM/SC-FDM) signal generators306. The transmitters 100 a and 100 b may transmit more than onecodeword. The scramblers 301 scramble the coded bits of each codeword,for transmission on a physical channel. A codeword may be referred to asa data stream and is equivalent to a data block from the MAC layer. Thedata block from the MAC layer is referred to as a transport block.

The modulation mappers 302 modulate the scrambled bits, thus producingcomplex modulation symbols. The modulation mappers 302 modulate thescrambled bits to complex modulation symbols representing positions on asignal constellation in a predetermined modulation scheme. Themodulation scheme may be, but not limited to, any of m-Phase ShiftKeying (m-PKS) and m-Quadrature Amplitude Modulation (m-QAM).

The layer mapper 303 maps the complex modulation symbols to one orseveral transmission layers.

The precoder 304 may precode the complex modulation symbols on eachlayer, for transmission through the antenna ports. More specifically,the precoder 304 generates antenna-specific symbols by processing thecomplex modulation symbols for multiple transmission antennas 500-1 to500-N_(t) in a MIMO scheme, and distributes the antenna-specific symbolsto the RE mappers 305. That is, the precoder 304 maps the transmissionlayers to the antenna ports. The precoder 304 may multiply an output xof the layer mapper 303 by an N_(t)×M_(t) precoding matrix W and outputthe resulting product in the form of an N_(t)×M_(F) matrix z.

The RE mappers 305 map/allocate the complex modulation symbols for therespective antenna ports to REs. The RE mappers 305 may allocate thecomplex modulation symbols for the respective antenna ports toappropriate subcarriers, and may multiplex them according to users.

The OFDM/SC-FDM signal generators 306 modulate the complex modulationsymbols for the respective antenna ports, that is, the antenna-specificsymbols through OFDM or SC-FDM modulation, thereby producing a complextime-domain OFDM or SC-FDM symbol signal. The OFDM/SC-FDM signalgenerators 306 may perform Inverse Fast Fourier Transform (IFFT) on theantenna-specific symbols and insert a Cyclic Prefix (CP) into theresulting IFFT time-domain symbol. The OFDM symbol is transmittedthrough the transmission antennas 500-1 to 500-N_(t) to a receiver afterdigital-to-analog conversion, frequency upconversion, etc. TheOFDM/SC-FDM signal generators 306 may include an IFFT module, a CPinserter, a Digital-to-Analog Converter (DAC), a frequency upconverter,etc.

If the transmitters 100 a and 100 b adopt SC-FDMA for transmitting acodeword, the transmitters 100 a and 100 b include an FFT processor (notshown). The FFT processor performs FFT on the complex modulation symbolsfor each antenna and outputs the FFT symbol to the RE mappers 305.

The receivers 300 a and 300 b operate in the reverse order to theoperation of the transmitters 100 a and 100 b. The receivers 300 a and300 b decode and demodulate radio signals received through the antennas500 a and 500 b from the outside and transfer the demodulated signals tothe processors 400 a and 400 b. The antenna 500 a or 500 b connected toeach of the receivers 300 a and 300 b may include N_(r) receptionantennas. A signal received through each reception antenna isdownconverted to a baseband signal and then recovered to the originaldata stream transmitted by the transmitter 100 a or 100 b throughmultiplexing and MIMO demodulation. Each of the receivers 300 a and 300b may include a signal recoverer for downconverting a received signal toa baseband signal, a multiplexer for multiplexing received signals, anda channel demodulator for demodulating the multiplexed signal stream toa codeword. The signal recoverer, the multiplexer, and the channeldecoder may be configured into an integrated module for performing theirfunctions or independent modules. To be more specific, the signalrecoverer may include an Analog-to-Digital Converter (ADC) forconverting an analog signal to a digital signal, a CP remover forremoving a CP from the digital signal, an FFT module for generating afrequency-domain symbol by performing FFT on the CP-removed signal, andan RE demapper/equalizer for recovering antenna-specific symbols fromthe frequency-domain symbol. The multiplexer recovers transmissionlayers from the antenna-specific symbols and the channel demodulatorrecovers the codeword transmitted by the transmitter from thetransmission layers.

If the receivers 300 a and 300 b receive SC-FDM signals, each of thereceivers 300 a and 300 b further includes an IFFT module. The IFFTmodule IFFT-processes the antenna-specific symbols recovered by the REdemapper and outputs the IFFT symbol to the multiplexer.

While it has been described in FIGS. 1 and 2 that each of thetransmitters 100 a and 100 b includes the scramblers 301, the modulationmappers 302, the layer mapper 303, the precoder 304, the RE mappers 305,and the OFDM/SC FDM signal generators 306, it may be furthercontemplated that the scramblers 301, the modulation mappers 302, thelayer mapper 303, the precoder 304, the RE mappers 305, and the OFDM/SCFDM signal generators 306 are incorporated into each of the processors400 a and 400 b of the transmitters 100 a and 100 b. Likewise, while ithas been described in FIGS. 1 and 2 that each of the receivers 300 a and300 b includes the signal recoverer, the multiplexer, and the channeldemodulator, it may be further contemplated that the signal recoverer,the multiplexer, and the channel demodulator are incorporated into eachof the processors 400 a and 400 b of the receivers 300 a and 300 b. Forthe convenience's sake of description, the following description will begiven with the appreciation that the scramblers 301, the modulationmappers 302, the layer mapper 303, the precoder 304, the RE mappers 305,and the OFDM/SC FDM signal generators 306 are included in thetransmitters 100 a and 100 b configured separately from the processors400 a and 400 b that controls their operations, and the signalrecoverer, the multiplexer, and the channel demodulator are included inthe receivers 300 a and 300 b configured separately from the processors400 a and 400 b that controls their operations. However, it is to benoted that even though the scramblers 301, the modulation mappers 302,the layer mapper 303, the precoder 304, the RE mappers 305, and theOFDM/SC FDM signal generators 306 are included in the processors 400 aand 400 b or the signal recoverer, the multiplexer, and the channeldemodulator are included in the processors 400 a and 400 b, embodimentsof the present invention are applicable in the same manner.

FIG. 3 illustrates an exemplary structure of a radio frame in a wirelesscommunication system. Specifically, the radio frame is a 3GPP LTE/LTE-Aradio frame. The radio frame structure is applicable to a FrequencyDivision Duplex (FDD) mode, a half FDD (H-FDD) mode, and a Time DivisionDuplex (TDD) mode.

Referring to FIG. 3, a 3GPP LTE/LTE-A radio frame is 10 ms (307,200 Ts)in duration. The radio subframe is divided into 10 equally-sizedsubframes, each subframe being 1 ms long. T_(s) represents a samplingtime and is given as T_(s)=1/(2048×15 kHz). Each subframe is furtherdivided into two slots, each of 0.5 ms in duration. 20 slots aresequentially numbered from 0 to 19. A time interval in which onesubframe is transmitted is defined as a Transmission Time Interval(TTI).

FIG. 4 illustrates an exemplary structure of a DownLink/UpLink (DL/UL)slot in the wireless communication system. Specifically, FIG. 4illustrates the structure of a resource grid in the 3GPP LTE/LTE-Asystem.

Referring to FIG. 4, a slot includes a plurality of OFDM symbols in thetime domain by a plurality of Resource Blocks (RBs) in the frequencydomain. An OFDM symbol may refer to one symbol duration. An RB includesa plurality of subcarriers in the frequency domain. An OFDM symbol maybe called an OFDM symbol, an SC-FDM symbol, etc. according to a multipleaccess scheme. The number of OFDM symbols per slot may vary depending ona channel bandwidth and a CP length. For instance, one slot includes 7OFDM symbols in case of a normal CP, whereas one slot includes 6 OFDMsymbols in case of an extended CP. While a subframe is shown in FIG. 4as having a slot with 7 OFDM symbols for illustrative purposes,embodiments of the present invention are also applicable to subframeswith any other number of OFDM symbols. A resource including one OFDMsymbol by one subcarrier is referred to as a Reference Element (RE) or atone.

Referring to FIG. 4, a signal transmitted in each slot may be describedby a resource grid including N^(DL/UL) _(RB)N^(RB) _(sc) subcarriers andN^(DL/UL) _(symb) OFDM or SC-FDM symbols. N^(DL) _(RB) represents thenumber of RBs in a DL slot and N^(UL) _(RB) represents the number of RBsin a UL slot. N^(DL) _(symb) represents the number of OFDM or SC-FDMAsymbols in the DL slot and N^(UL) _(symb) represents the number of OFDMor SC-FDMA symbols in the UL slot. N^(RB) _(sc) represents the number ofsubcarriers in one RB.

In other words, a Physical Resource Block (PRB) is defined as N^(DL/UL)_(symb) consecutive OFDM symbols or SC-FDMA symbols in the time domainby N^(RB) _(sc) consecutive subcarriers in the frequency domain.Therefore, one PRB includes N^(DL/UL) _(symb)×N^(RB) _(sc) REs.

Each RE in the resource grid may be uniquely identified by an index pair(k, l) in a slot. k is a frequency-domain index ranging from 0 toN^(DL/UL) _(RB)×N^(RB) _(sc)−1 and l is a time-domain index ranging from0 to N^(DL/UL) _(symb)−1.

FIG. 5 illustrates an exemplary structure of a DL subframe in thewireless communication system.

Referring to FIG. 5, each subframe may be divided into a control regionand a data region. The control region includes one or more OFDM symbols,starting from the first OFDM symbol. The number of OFDM symbols used forthe control region of a subframe may be set independently on a subframebasis and signaled on a PCFICH. A BS may transmit control information toa UE or UEs in the control region. To transmit control information, aPDCCH, a PCFICH, a PHICH, etc. may be allocated to the control region.

The BS may transmit data to a UE or UE group in the data region. Datatransmitted in the data region is referred to as user data. A PDSCH maybe allocated to the data region to convey data. A UE may decode controlinformation received on a PDCCH and thus read data received on a PDSCHbased on the decoded control information. For example, the PDCCH carriesinformation indicating a UE or UE group to which the data of the PDSCHis destined and information indicating how the UE or UE group shouldreceive and decode the PDSCH data.

The PDCCH delivers information about the transport format and resourceallocation of a DownLink Shared CHannel (DL-SCH), resource allocationinformation about an UpLink Shared CHannel (UL-SCH), paging informationabout a Paging CHannel (PCH), system information about the DL-SCH,allocation information of a higher-layer control message such as arandom access response transmitted on a PDSCH, a collection ofTransmission Power Control (TPC) commands for the UEs of a UE group,activation information about Voice over Internet Protocol (VoIP), etc. Aplurality of PDCCHs may be transmitted in the control region. A UE maydetect its own PDCCH by monitoring the plurality of PDCCHs. The size andusage of control information transmitted on a PDCCH may vary accordingto a Downlink Control Information (DCI) format and the size of thecontrol information may vary according to coding rates.

An independent DCI format applies to each UE and PDCCHs for a pluralityof UEs may be multiplexed in one subframe. The PDCCH of each UE isindependently channel-encoded and added with a Cyclic Redundancy Check(CRC). The CRC is masked by a unique ID of the UE so that the UE mayreceive its own PDCCH. Basically, however, without knowledge of theposition of its own PDCCH, the UE performs blind detection (or blinddecoding) on all PDCCHs with a specific DCI format until it receives aPDCCH having its ID.

Various types of RSs are transmitted between a BS and a UE for thepurposes of interference mitigation, estimation of the channel statebetween the BS and the UE, demodulation of signals transmitted betweenthe BS and the UE, etc. An RS refers to a predefined signal with aspecial waveform known to both the BS and the UE, transmitted from theBS to the UE or from the UE to the BS.

RSs are largely classified into DRSs and CRSs. CRSs are transmitted inevery DL subframe in a cell supporting PDSCH transmission. CRSs are usedfor both purposes of demodulation and measurement and shared among allUEs within the cell. A CRS sequence is transmitted through every antennaport irrespective of the number of layers. DRSs are usually used fordemodulation, dedicated to a specific UE. The CRSs and DRSs are alsocalled cell-specific RSs and DMRSs, respectively. The DMRSs are alsocalled UE-specific RSs.

FIG. 6 is a conceptual view of DRS (i.e. DMRS) transmission.Particularly, a transmitter for transmitting precoded RSs is illustratedin FIG. 6, by way of example.

A DRS is dedicated to a particular UE and thus other UEs are not allowedto use the DRS. DRSs used for data demodulation at a specific UE may beclassified into precoded RSs and non-precoded RSs. If precoded RSs areused as DRSs, the DRSs are precoded with a precoding matrix used forprecoding data symbols, and as many RS sequences as K layers aretransmitted. K is equal to or less than the number of physical antennaports, N_(t). The K layers may be allocated to one or more UEs. If aplurality of UEs share the K layers, 1 UE to K UEs may receive the Klayers in the same time/frequency resources.

The UE may demodulate a received data signal by arranging the datasignal at predetermined positions on a signal constellation according toa predetermined modulation scheme, based on DRSs received along with thedata signal.

FIG. 7 illustrates exemplary DRS patterns in an LTE system.Specifically, FIG. 7( a) illustrates a DRS pattern for a subframe with anormal CP and FIG. 7( b) illustrates a DRS pattern for a subframe withan extended CP. In FIG. 7, ‘1’ represents the position of an OFDM symbolin a slot.

REs on which DRSs can be transmitted (i.e. DRS REs) are generallypreset, among the REs of an RB or an RB pair. Thus, a UE has only todetect a DRS(s) from an RE(s) at a preset position(s) among the REs ofeach RB or RB pair. For example, referring to FIG. 7, a BS transmitsDRSs in one or more RB pairs through antenna port 5 in the pattern ofFIG. 7( a) or 7(b). Hereinbelow, the positions of DRS REs in an RB or RBpair will be referred to as a DRS pattern in describing embodiments ofthe present invention.

In the LTE system supporting up to two layers, a BS simultaneouslytransmits DRSs for demodulation of the layers and CRSs for estimation ofa channel between a UE and the BS. RSs should be transmitted through allphysical antenna ports in CRS-based downlink transmission. Therefore,the CRS-based downlink transmission faces the problem that overall RSoverhead increases with the number of physical antenna ports and thusdata transmission efficiency is decreased. To avert this problem, theLTE-A system that can transmit more layers than the LTE system uses DRSsfor demodulation instead of CRSs that increase transmission overheadaccording to the number of physical antenna ports. In DRS-based downlinktransmission, only virtual antenna ports need RSs for coherentdemodulation. That is, only virtual antenna ports, not all physicalantenna ports of the BS, transmit their DRSs in the DRS-based downlinktransmission. Since the number of virtual antenna ports is generallysmaller than or equal to the number of physical antenna ports, N_(t),the DRS-based downlink transmission advantageously decreases RStransmission overhead, compared to the CRS-based downlink transmission.

Since DRSs precoded in the same precoder as used for data serve only thepurpose of demodulation, measurement RSs called CSI-RSs are additionallytransmitted to allow UEs to measure channel states in the LTE-A system.Because channel states do not change much over time, CSI-RSs aretransmitted at every predetermined interval of a plurality of subframes,compared to CRSs transmitted in every subframe. In view of thetransmission nature of CSI-RSs, the transmission overhead of the CSI-RSsis smaller than that of the CRSs.

According to the present invention, DRSs are used for PDSCH transmissionand as many DRSs as the number of layers used for the PDSCH transmissionare transmitted for demodulation of the layers. The DRSs are transmittedonly in RBs to which the PDSCH is mapped. In addition, the DRSs are nottransmitted in REs used for other types of RSs irrespective of antennaports.

FIG. 8 illustrates an exemplary DRS pattern in the LTE-A system.Specifically, the DRS pattern is for an RB pair in a regular subframewith a normal CP.

In the LTE-A system, a plurality of layers may be multiplexed in asubframe, prior to transmission to a UE. Because DRSs should betransmitted for the respective layers, the number of DRSs increases inproportion to the number of transmitted layers. If a plurality of DRSsare transmitted in different REs, the number of DRS REs increases withthe number of layers, thereby decreasing data transmission efficiency.Therefore, when a plurality of DRSs are to be transmitted, one or moreDRSs are preferably multiplexed in a predetermined RE in order todecrease DRS transmission overhead.

Therefore, a plurality of DRSs are transmitted largely in two groups ofREs in the LTE-A system. For instance, one or more DRSs may bemultiplexed in REs labeled with “C” and one or more other DRSs may bemultiplexed in REs labeled with “D”, for transmission to UEs in FIG. 8.When a plurality of DRSs are multiplexed in a predetermined radioresource, the DRSs may be distinguished from one another by theirOrthogonal Cover Codes (OCCs). For instance, up to two different DRSsmay be transmitted in a single RE by extending the DRSs using OCCs oflength 2. In another example, up to four different DRSs may betransmitted in a single RE by extending the DRSs using OCCs of length 4.The OCCs may be, for example, Walsh-Hadamard codes. An OCC is alsocalled an orthogonal sequence.

Hereinbelow, a set of REs carrying DRSs which are extended by OCCs andthus distinguishable from one another among the REs of an RB or RB pairare referred to as a Code Division Multiplexing (CDM) group. Referringto FIG. 8, REs labeled with “C” form one CDM group (CDM group 1) and REslabeled with “D” form another CDM group (CDM group 2). In a pair ofsuccessive RBs (i.e. an RB pair) in a subframe, each CDM group includes12 REs in FIG. 8.

FIG. 9 illustrates exemplary patterns of multiplexing DRSs for twolayers in a subframe with a normal CP, using OCCs of length 2.

Referring to FIGS. 9( a), 9(b) and 9(c), two DRSs for two layers aremapped to radio resources in the following manner. For instance, it isassumed that virtual antenna ports mapped to the two layers in aone-to-one correspondence are antenna port 7 and antenna port 8. In asubframe with a normal CP, a part of each of DRS sequences r(m) forantenna ports 7 and 8 may be mapped to complex-valued modulationsymbols, a^((p)) _(k,l) in a Physical Resource Block (PRB) with afrequency domain index n_(PRB), allocated for transmission of a PDSCHaccording to the following Formula.

$\begin{matrix}{{a_{k,l}^{(p)} = {s \cdot {r( {{3 \cdot l^{\prime} \cdot N_{RB}^{\max,{DL}}} + {3 \cdot n_{PRB}} + m^{\prime}} )}}}{where}{s = \{ {{\begin{matrix}1 & {{{if}\mspace{14mu} p} = 7} \\( {- 1} )^{m^{\prime} + l^{\prime} + n_{PRB}} & {{{if}\mspace{14mu} p} = 8}\end{matrix}k} = {{{5 \cdot m^{\prime}} + m^{\prime} + {N_{sc}^{RB} \cdot n_{PRB}} + {1l}} = \{ {{\begin{matrix}{{l^{\prime}{mod}\; 2} + 2} & {{{if}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 3},4,{{or}\mspace{14mu} 8}} \\{{l^{\prime}{mod}\; 2} + 2 + {3 \cdot \lfloor {l^{\prime}/2} \rfloor}} & {{{if}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 1},2,6,{{or}\mspace{14mu} 7}} \\{{l^{\prime}{mod}\; 2} + 5} & {{if}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}}\end{matrix}l^{\prime}} = \{ {{{\begin{matrix}{0,1,2,3} & {{{{if}\mspace{14mu} n_{s}{mod}\; 2} = {0\mspace{14mu}{and}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 1}},2,6,{{or}\mspace{14mu} 7}} \\{0,1} & {{{{if}\mspace{14mu} n_{s}{mod}\; 2} = {0\mspace{14mu}{and}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 1}},2,6,{{or}\mspace{14mu} 7}} \\{2,3} & {{{{if}\mspace{14mu} n_{s}{mod}\; 2} = {1\mspace{14mu}{and}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu} 1}},2,6,{{or}\mspace{14mu} 7}}\end{matrix}m^{\prime}} = 0},1,2} } }} }} & \lbrack {{Formula}\mspace{14mu} 1} \rbrack\end{matrix}$

In [Formula 1], p is the index of an antenna port, satisfying pε{7, 8},k and l are a subcarrier index and an OFDM symbol index described beforewith reference to FIG. 4, r(s) is a random sequence, and m′ is a counterof DRS REs in each OFDM symbol used for the PDSCH transmission. Sinceeach DRS OFDM symbol includes three DRS subcarriers for each RB, m′ isone of 0, 1 and 2. N^(max,DL) _(RB) is the maximum number of RBs in a DLslot allocated to the PDSCH, n_(s) is a slot number in a radio frame,and l′ is a counter of DRS OFDM symbols in a subframe. In a normalsubframe other than a special subframe, there are a total of four DRSOFDM symbols in case of a normal CP. Hence, l′ is one of 0, 1, 2 and 3.

Referring to FIG. 9, a DRS for a layer corresponding to antenna port 7and a DRS for a layer corresponding to antenna port 8 are transmitted inthe same REs. In other words, the DRSs of antenna port 7 and antennaport 8 are multiplexed in a predetermined CDM group, for example, CDMgroup 1.

The following description will be given of embodiments of the presentinvention in the context of a normal subframe with a CDM groupconfigured as illustrated in FIG. 9( a). However, it is to be clearlyunderstood that the present invention is applicable to a specialsubframe as well as a normal subframe in the same manner.

FIG. 10 illustrates an exemplary transmission of DRSs for four layers intwo CDM groups. When two CDM groups are used, two DRSs may bemultiplexed in each CDM group, for transmission.

Two DRSs may be multiplexed in one RE, using OCCs of length 2 (OCC=2)and the number of layers that can be multiplexed for transmissionincreases in proportion to the number of CDM groups. For example, up tofour DRS sequences may be transmitted in two CDM groups, using OCCs oflength 2 (OCC=2).

FIG. 11 illustrates a method for multiplexing four DRSs in two CDMgroups. In a MIMO system supporting a maximum rank of 4, up to four DRSsequences can be transmitted in two CDM groups. That is, two DRSs can bemultiplexed in each CDM group, using two OCC sequences of length 2(OCC=2).

Referring to FIG. 11, it is assumed that virtual antenna portscorresponding to DRSx, DRSy, DRSz and DRSw are DRS port X, DRS port Y,DRS port Z, and DRS port W, respectively, and two OCC sequences oflength 2 are [1 1] and [1 −1]. The two OCC sequences are therow-direction sequences of a 2×2 matrix illustrated in FIG. 11.

In FIG. 11, DRSx and DRSy may be extended by the sequences [1 1] and [1−1], respectively and then allocated to CDM group 1. DRSz may beextended by one of the sequences [1 1] and [1 −1], DRSw may be extendedby the other sequence, and then the extended DRSz and DRSw may beallocated to CDM group 2.

An RB pair illustrated in FIG. 11 includes a total of four DRS symbols,DRS symbol 1 to DRS symbol 4. A part of DRSx extended by the sequence [11] and a part of DRSy extended by the sequence [1 −1] are allocated toDRS symbol 1. For example, DRSx is extended to [DRSx DRSx] bymultiplying DRSx by [1 1] and DRSy is extended to [DRSy −DRSy] bymultiplying DRSy by [1 −1]. The first elements of the extended DRSx andDRSy, DRSx and DRSy may be allocated to DRS symbol 1 and the secondelements of the extended DRSx and DRSy, DRSx and −DRSy may be allocatedto DRS symbol 2. That is, (1×DRSx)+(1×DRSy) is allocated to DRS symbol 1and (1×DRSx)+(−1×DRSy) is allocated to DRS symbol 2.

In summary, four DRSs may be allocated to DRS REs in two CDM groups,using OCCs as illustrated in [Table 1].

TABLE 1 Orthogonal Cover Code DRS port [w_(p)(0) w_(p)(1)] CDM group 0[+1 +1] 1 1 [+1 −1] 1 2 [+1 +1] 2 3 [+1 −1] 2

Referring to [Table 1], DRS ports are mapped to layers in a one-to-onecorrespondence. Thus, the indexes of DRS ports may be used as theindexes of layers or vice versa. Antenna port 7 to antenna port 10 maybe mapped to DRS port 0 to DRS port 3 in a one-to-one correspondence. ADRS for each DRS port is extended by [w_(p)(0) w_(p)(1)] and mapped toone pair of REs in its CDM group.

For each DM group, DRS ports allocated to the CDM group and OCCs used tospread DRSs for the DRS ports are listed in [Table 2] below.

TABLE 2 CDM group 1 CDM group 2 DRS Orthogonal Cover Code OrthogonalCover Code port [w_(p)(0) w_(p)(1)] DRS port [w_(p)(0) w_(p)(1)] 0 [+1+1] 2 [+1 +1] 1 [+1 −1] 3 [+1 −1]

Referring to [Table 1] or [Table 2], (+1×DRS0)+(+1×DRS1) and(+1×DRS0)+(−1×DRS1) are sequentially mapped to REs of CDM group 1 and(+1×DRS2 )+(+1×DRS3) and (+1×DRS2)+(−1×DRS3) are sequentially mapped toREs of CDM group 2.

OCCs of length 2 used for spreading DRSs and OCCs of length 2 used formultiplexing the DRSs in one RE may be simply expressed as the followingFormula.

$\begin{matrix}\begin{matrix}{W_{2} = \begin{pmatrix}{+ 1} & {+ 1} \\{+ 1} & {- 1}\end{pmatrix}} \\{= \begin{pmatrix}a & b\end{pmatrix}} \\{= \begin{pmatrix}x \\y\end{pmatrix}}\end{matrix} & \lbrack {{Formula}\mspace{14mu} 2} \rbrack\end{matrix}$

In [Formula 2], the column vectors a and b are OCCs each being used formultiplexing a plurality of DRSs. Each of the column vectors a and b arecomprised of coefficients that will be multiplied with DRSs. Each of therow vectors x and y represents an OCC that spreads a DRS, that is, eachrow vector is a kind of spreading factor. Hereinbelow, an OCC seen fromthe viewpoint of spreading is referred to as a spreading OCC and an OCCseen from the viewpoint of multiplexing is referred to as a multiplexingOCC in describing the present invention.

For the convenience′ sake of description, a form in which a plurality ofDRSs are multiplexed in an RE is expressed with row a (representing thecolumn vector a of weights multiplied with DRSs) or b (representing thecolumn vector b of weights multiplied by the DRSs). For instance, inFIG. 12 illustrating examples of multiplexing two DRSs into one CDMgroup, RE ‘a’ represents an RE which two DRSs are multiplexed by theelements of the column vector a and allocated thereto, and RE ‘b’represents an RE which two DRSs are multiplexed by the elements of thecolumn vector b and allocated thereto.

Referring to FIG. 12, in the case where multiplexing OCCs are allocated,multiplexing OCCs used for multiplexing layers in an OFDM symbol arepreset. Even though a plurality of RBs are allocated to a UE, themultiplexing OCCs are allocated to the RBs in the same pattern. Becauselayers transmitted to the UE are scrambled with the same scramblingsequence, the DRS allocation illustrated in FIG. 12 may causeconcentration of transmission power on a specific OFDM symbol.Therefore, transmission power efficiency may be decreased. It ispreferable that transmission power of a BS is uniform within a maximumtransmission power range irrespective of time in order to increase thedata transmission rate of the BS. Accordingly, there exists a need forappropriately allocating multiplexing OCCs to prevent powerconcentration on a specific OFDM symbol, that is, to uniformlydistribute transmission power across OFDM symbols.

FIG. 13 illustrates an embodiment of the present invention for uniformlydistributing transmission power across OFDM symbols in rank-2transmission.

Referring to FIG. 13, to prevent the counterbalance between DRSsequences or too large a sum of the DRS sequences in specific OFDMsymbols, the allocation positions of multiplexing OCCs may be swapped orshifted in the time domain or the frequency domain.

FIGS. 14 and 15 illustrate exemplary power allocations to DRS REs anddata REs in rank-2 transmission.

Referring to FIG. 14, given a rank of 2, a BS may transmit two layersand two DRSs through two DRS ports. Since the same precoder is used forprecoding the two layers and the two DRSs, the power ratio between dataREs and DRS REs is the same for each layer.

Therefore, a UE can determine the power ratio between data REs and DRSREs for each layer without receiving additional information from the BS.It is because as each DRS port transmits a signal allocated to a data REand a signal allocated to a DRS RE with the same power, the power ratiobetween the data RE and the DRS RE is implicitly signaled to the UE.Thus, different layers may have different power ratios in rank-2transmission. Referring to FIG. 15, layer 0 and layer 1 may betransmitted at different power levels.

Referring to FIGS. 14 and 15, a transmission power per RE as well as atransmission power for each layer in each RE is constant. That is, powermay be uniformly distributed across OFDM symbols in a subframe in up torank-2 transmission. However, in rank-3 or higher-rank transmission, thenumber of layers per data RE and the number of layers per DRS RE mayvary in an OFDM symbol according to the length of OCCs and the number ofCDM groups. For example, referring to FIG. 10, on the assumption that atotal of four layers are mapped to antenna port 7 to antenna port 10 ina one-to-one correspondence, the four layers are multiplexed in eachdata RE, whereas two DRSs are multiplexed in each DRS RE. As aconsequence, a data RE and a DRS RE may have different transmissionpower per layer in rank-3 or higher-rank transmission. Hence, uniformpower distribution to OFDM symbols in a subframe may be more difficultin rank-3 or higher-rank transmission than in rank-2 or lower-ranktransmission. Accordingly, a power balancing scheme should be specifiedto prevent fluctuation of transmission power over OFDM symbols in theLTE-A system supporting rank-3 or higher-rank transmission.

A method for allocating/configuring DRSs in such a manner that power canbe uniformly distributed to OFDM symbols in a subframe will be describedbelow. For the convenience' sake of description, the present inventionwill be described, taking an example of using two CDM groups to supportup to eight layers.

To transmit DRSs for eight layers in two CDM groups, OCCs of length 4(OCC=4) may be used as illustrated in FIG. 15. FIG. 16 illustrates anexample of allocating DRSs for layers corresponding to antenna port 11to antenna port 14 in two CDM groups. Referring to FIGS. 10 and 16, itis noted that DRSs for layers corresponding to antenna port 7 to antennaport 14 are multiplexed in fours in two CDM groups. Namely, each CDMgroup carries up to four DRSs.

FIG. 17 illustrates a method for multiplexing eight DRSs in two CDMgroups.

In a MIMO system supporting a rank of up to 8, up to eight DRS sequencesmay be transmitted in two CDM groups. Four DRSs may be multiplexed ineach CDM group, using four OCC sequences of length 4. Let virtualantenna ports transmitting DRSx, DRSy, DRSz and DRSw be DRS port X, DRSport Y, DRS port Z, and DRS port W, respectively. It is assumed that theOCC sequences of length 4 are [1 1 1 1], [1 −1 1 −1], [1 1 −1 −1], and[1 −1 −1 1], respectively. The four OCC sequences correspond to therow-direction sequences of a 4×4 matrix illustrated in FIG. 17.

Referring to FIG. 17, DRSx is extended by the sequence [1 1 1 1], DRSyis extended by the sequence [1 −1 1 −1], DRSz is extended by thesequence [1 1 −1 −1], and DRSw is extended by the sequence [1 −1 −1 1].Then the extended DRSx, DRSy, DRSz, and DRSw may be allocated to CDMgroup 1. Four DRSs other than DRSx, DRSy, DRSz, and DRSw are extended bythe respective sequences, [1 1 1 1], [1 −1 1 −1], [1 1 −1 −1], and [1 −1−1 1], and then may be allocated to CDM group 2.

In FIG. 17, an RB pair includes four DRS symbols, DRS symbol 1 to DRSsymbol 4. Parts of DRSx, DRSy, DRSz, and DRSw extended by the sequences[1 1 1 1], [1 −1 1 −1], [1 1 −1 −1], and [1 −1 −1 1], respectively areallocated to DRS symbol 1. For example, DRSx is extended to [DRSx DRSxDRSx DRSx] by multiplying DRSx by the sequence [1 1 1 1], DRSy isextended to [DRSy −DRSy DRSy −DRSy] by multiplying DRSy by the sequence[1 −1 1 −1], DRSz is extended to [DRSz DRSz −DRSz −DRSz] by multiplyingDRSx by the sequence [1 1 −1 −1], and DRSw is extended to [DRSw −DRSw−DRSw DRSw] by multiplying DRSw by the sequence [1 −1 −1 1]. Forexample, the first elements DRSx, DRSy, DRSz and DRSw of the extendedDRS sequences are allocated to DRS symbol 1, the second elements DRSx,−DRSy, DRSz and −DRSw of the extended DRS sequences are allocated to DRSsymbol 2, the third elements DRSx, DRSy, −DRSz and −DRSw of the extendedDRS sequences are allocated to DRS symbol 3, and the fourth elementsDRSx, −DRSy, −DRSz and DRSw of the extended DRS sequences are allocatedto DRS symbol 4. That is, a (1×DRSx)+(1×DRSy)+(1×DRSz)+(1×DRSw) elementis allocated to DRS symbol 1, a (1×DRSx)+(−1×DRSy)+(1×DRSz)+(−1×DRSw)element is allocated to DRS symbol 2, a(1×DRSx)+(1×DRSy)+(−1×DRSz)+(−1×DRSw) element is allocated to DRS symbol3, and a (1×DRSx)+(−1×DRSy)+(−1×DRSz)+(1×DRSw) element is allocated toDRS symbol 4.

In summary, four DRSs may be allocated to DRS REs of two CDM groups,using the following OCCs.

$\begin{matrix}\begin{matrix}{W_{4} = \begin{pmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {+ 1} & {- 1} & {- 1} \\{+ 1} & {- 1} & {- 1} & {+ 1}\end{pmatrix}} \\{= \begin{pmatrix}a & b & c & d\end{pmatrix}} \\{= \begin{pmatrix}x \\y \\z \\w\end{pmatrix}}\end{matrix} & \lbrack {{Formula}\mspace{14mu} 3} \rbrack\end{matrix}$

In [Formula 3], the column vectors a, b, c and d are multiplexing OCCseach including coefficients that are multiplied with a plurality ofDRSs, when the DRSs are multiplexed into one RE. Each of the row vectorsx, y, z and w is a spreading OCC with which a DRS is spread. For theconvenience' sake of description, DRSs multiplexed in one RE arerepresented by a multiplexing OCC having weights multiplied with theDRSs in describing the present invention.

The DRS multiplexing illustrated in FIG. 11 and the DRS multiplexingillustrated in FIG. 17 may be carried out simultaneously or only eitherof them may be carried out in a wireless communication system. Forexample, the DRS multiplexing scheme of FIG. 11 may be used for a BS tomultiplex one to four layers, for transmission, whereas the DRSmultiplexing scheme of FIG. 17 may be used for a BS to multiplex five toeight layers, for transmission. In another example, one to eight layersmay be multiplexed and transmitted using the DRS multiplexing scheme ofFIG. 17. Notably, since the length of OCCs varies with the total numberof layers transmitted by a BS in the former case, information indicatingthe total number of layers transmitted by the BS or the length of OCCsused for multiplexing the layers should be signaled to a UE explicitlyor implicitly so that the UE may detect its layer using an OCC.

Now a description will be given of embodiments of the present inventionfor uniformly distributing transmission power across OFDM symbolsaccording to scenarios.

<One CDM Group and OCC=4 Allocation>

FIGS. 18 to 22 illustrate multiplexing of DRSs in one CDM group usingOCCs of length 4 according to embodiments of the present invention.

Embodiment 1

Referring to FIG. 18, four multiplexing OCCs may be allocated to DRSOFDM symbols on each DRS subcarrier in the same order in every RB, suchthat from the viewpoint of one DRS OFDM symbol, only one of the OCCs isused in the DRS OFDM symbol. Therefore, power may concentrate on one DRSOFDM symbol. In the case where four multiplexing OCCs of length 4 areallocated to a CDM group, Embodiment 2 to Embodiment 5 may becontemplated for uniform power distribution across OFDM symbols.

Embodiment 2

Referring to FIG. 19, the OCCs a and b of an OCC group (a, b) may beswapped with each other on DRS subcarriers in a slot, and the OCCs c andd of an OCC group (c, d) may be swapped with each other on DRSsubcarriers in another slot. Specifically, if the OCCs a and b areallocated in the order of [a b] to a DRS subcarrier in a slot, the OCCsa and b are allocated in the reverse order of [b a] to the next DRSsubcarrier in the slot. Since the order of two OCCs alternates betweenforward and reverse on DRS subcarriers in a slot, the same OCCallocation pattern occurs in every successive RB pair. Therefore, eventhough a plurality of RBs are allocated to a particular UE, OCCs areallocated in the same pattern in every two PRBs. One thing to noteherein is that from the viewpoint of one DRS OFDM symbol, only two offour OCCs are used. From the viewpoint of one RB, only two OCCs are usedfor the RB. This means that the OCCs are not uniformly distributedacross a subframe. Thus it may be concluded that OCC allocationaccording to Embodiment 2 makes it difficult to uniformly distributepower across all OFDM symbols in a subframe.

Embodiment 3-1

In Embodiment 3-1, the OCC patterns of two slots in an RB are swappedwith each other in the next RB so that all OCCs exist in an OFDM symbol.Referring to FIG. 20( a), even though all OCCs are not allocated in anRB, all OCCs exist in an OFDM symbol. Thus, Embodiment 3-1 may achieve auniform power distribution across OFDM symbols in a subframe, comparedto Embodiment 2.

However, because only a part of the OCCs are allocated to an RB, powerfluctuates over different RBs in the frequency domain.

Embodiment 3-2

For uniform distribution of OCCs at an RB level, four OCCs are allocatedto each of DRS subcarriers in a reverse order to an OCC allocation orderof the previous DRS subcarrier in a subframe. Referring to FIG. 20( b),if OCCs a, b, c and d are allocated to a first DRS subcarrier of asubframe in the order of [a b c d], they are allocated to a second DRSsubcarrier of the subframe in the reverse order to [a b c d], that is,in the order of [d c b a], and to a third DRS subcarrier of the subframein the reverse order to [d c b a], that is, in the order of [a b c d].

Embodiment 3-2 is advantageous in that all OCCs are allocated in oneslot. However, only two OCCs are repeated in each DRS OFDM symbol.

Embodiment 4

The OCCs (a, b, c, d) are cyclically shifted on DRS subcarriers in oneCDM group so that the OCCs may be uniformly distributed at both an RBlevel and a DRS OFDM symbol level. Referring to FIG. 21, the OCCs (a, b,c, d) are allocated in the order of [a b c d] to a first DRS subcarrierof RB #n in a subframe, in the order of [b c d a] to a second DRSsubcarrier of RB #n in the subframe by cyclically shifting [a b c d],and then in the order of [c d a b] to a third DRS subcarrier of RB #n inthe subframe by cyclically shifting [b c d a].

According to Embodiment 4, all OCCs are allocated to an RB as well as aDRS OFDM symbol. However, the four OCCs cannot be allocated to aresource area defined as one DRS OFDM symbol by one RB because onlythree DRS REs are available in the resource area.

Embodiment 5

The OCCs are cyclically shifted on DRS REs of DRS OFDM symbols in such amanner that OCCs are uniformly distributed across a plurality of RBs ineach DRS OFDM symbol. Referring to FIG. 22, four DRS OFDM symbolsinclude DRSs in a subframe and three DRS REs per RB are available ineach DRS OFDM symbol. Since different DRS OFDM symbols start withdifferent OCCs, all OCCs exist on each DRS subcarrier in a subframe.

<2 CDM Groups and OCC=4 Allocation>

FIGS. 23 to 30 illustrate multiplexing of DRSs in two CDM groups usingOCCs of length 4 according to embodiments of the present invention. Inthe case where four multiplexing OCCs of length 4 are allocated to eachof two CDM groups, the following embodiments may be considered touniformly distribute power across OFDM symbols. Embodiment 6, Embodiment7 and Embodiment 8 may be used in combination with any of Embodiment 1to Embodiment 5.

Embodiment 6

The simplest method for allocating four OCCs to each of two CDM groupsis to repeat the OCC allocation pattern of one CDM group for the otherCDM group. For example, referring to FIGS. 21 and 23, if Embodiment 4 isadopted for CDM group 1, the OCCs may be allocated to CDM group 2 in thesame OCC pattern of CDM group 1.

When OCCs are allocated to two adjacent DRS subcarriers according toEmbodiment 5, spreading OCCs and DRS ports for CDM group 1 and CDM group2 are placed in the following relationship.

TABLE 3 Orthogonal Cover Code DRS port [w_(p)(0) w_(p)(1) w_(p)(2)w_(p)(3)] CDM group 0 [+1 +1 +1 +1] 1 1 [+1 −1 +1 −1] 1 2 [+1 +1 +1 +1]2 3 [+1 −1 +1 −1] 2 4 [+1 +1 −1 −1] 1 5 [+1 +1 −1 −1] 2 6 [+1 −1 −1 +1]1 7 [+1 −1 −1 +1] 2

In [Table 3], DRS ports are virtual antenna ports that transmit DRSsamong antenna ports. The DRS ports one-to-one correspond to layers. Forexample, antenna port 7 to antenna port 14 may be mapped to DRS port 0to DRS port 7 , respectively. DRS port 0 to DRS port 7 may one-to-onecorrespond to layer 0 to layer 7 in [Table 3]. In this case, a spreadingOCC for each DRS port is a spreading OCC for each layer. A DRS for eachDRS port (or each layer) is extended by [w_(p)(0) w_(p)(1) w_(p)(2)w_(p)(3)] and mapped to four successive DRS REs on a DRS subcarrier in aCDM group corresponding to the DRS.

For the two CDM groups, DRS ports allocated to the CDM groups on two DRSsubcarriers and spreading OCCs used for layers corresponding to the DRSports are listed in [Table 4] below.

TABLE 4 CDM group 1 CDM group 2 DRS Orthogonal Cover Code DRS OrthogonalCover Code port [w_(p)(0) w_(p)(1) w_(p)(2) w_(p)(3)] port [w_(p)(0)w_(p)(1) w_(p)(2) w_(p)(3)] 0 [+1 +1 +1 +1] 2 [+1 +1 +1 +1] 1 [+1 −1 +1−1] 3 [+1 −1 +1 −1] 4 [+1 +1 −1 −1] 5 [+1 +1 −1 −1] 6 [+1 −1 −1 +1] 7[+1 −1 −1 +1]

In [Table 4], w_(p)(l′) is a weight multiplied by a layer in DRS OFDMsymbol l′. A vector of weights applied to the DRS ports of a CDM groupmay be regarded as a multiplexing OCC. For example, referring to [Table4], w_(p)(0) for DRS ports 0, 1, 4, and 6 allocated to CDM group 1 andw_(p)(0) for DRS ports 2, 3, 5 and 7 allocated to CDM group 2 are +1,+1, +1, +1. Therefore, a multiplexing OCC allocated to a start DRS OFDMsymbol on a start DRS subcarrier of CDM group 1 is a sequence a, [+1 +1+1 +1]. Referring to FIG. 23, multiplexing OCCs are allocated in theorder of [a b c d] to four DRS OFDM symbols on the start DRS subcarrierof each CDM group.

Embodiment 7

The OCC allocation pattern of a first CDM group is cyclically shiftedslotwise by two OCCs and then allocated to a second CDM group. Referringto FIG. 24( a), the OCCs (a, b, c, d) allocated to a DRS subcarrier arecyclically shifted by one OCC on the next DRS subcarrier in CDM group 1according to Embodiment 4. The OCC pattern of a DRS subcarrier of CDMgroup 1 is shifted by two OCCs slotwise on a DRS subcarrier of CDM group2 adjacent to the DRS subcarrier of CDM group 1. Therefore, if one CDMgroup starts with an OCC pattern [a b c d] over DRS OFDM symbols, theother CDM group starts with an OCC pattern [c d a b] in Embodiment 7.

Embodiment 8

The OCC allocation pattern of a first CDM group is cyclically shiftedslotwise by one OCC and then allocated to a second CDM group. Referringto FIG. 25( a), the OCCs (a, b, c, d) allocated to a DRS subcarrier arecyclically shifted by one OCC on the next DRS subcarrier in CDM group 1according to Embodiment 4. In CDM group 2, the OCC pattern of a DRSsubcarrier of CDM group 1 adjacent to a DRS subcarrier is shifted by oneOCC slotwise on the DRS subcarrier. Therefore, if one CDM group startswith an OCC pattern [a b c d] over DRS OFDM symbols, the other CDM groupstarts with an OCC pattern [d a b c] in Embodiment 8.

The OCC patterns illustrated in FIGS. 24( a) and 25(a) may be swapped inthe two CDM groups. FIGS. 24( b) and 25(b) illustrate embodiments inwhich the OCC patterns of CDM group 1 are swapped with the OCC patternsof CDM group 2 in FIGS. 24( a) and 25(a).

The same scrambling sequence may be applied to all DRS ports, ordifferent scrambling sequences may be applied to different DRS portgroups and/or different DRS ports in Embodiment 1 to Embodiment 8.

FIGS. 23, 24 and 25 illustrate methods for allocating OCCs to two CDMgroups, when OCCs are allocated to a first CDM group according toEmbodiment 4 as illustrated in FIG. 21, according to Embodiment 6,Embodiment 7, and Embodiment 8. If the OCCs are allocated to the firstCDM group according to Embodiment 5, the two CDM groups may have OCCpatterns illustrated in FIGS. 26, 27 and 28 according to Embodiment 6,Embodiment 7, and Embodiment 8. The OCC patterns of CDM group 1illustrated in FIGS. 26, 27 and 28 may be swapped with those of CDMgroup 2.

In Embodiment 7, in the case where OCCs are allocated to two CDM groups,if the OCCs are allocated in the order of [a b c d] to a DRS subcarrierin one CDM group, a 2-OCC shift version of the OCC pattern [a b c d],that is, [c d a b] is allocated to a DMRS subcarrier of the other CDMgroup, adjacent to the DRS subcarrier of the one CDM group. InEmbodiment 8, in the case where OCCs are allocated to two CDM groups, ifthe OCCs are allocated in the order of [a b c d] to a DRS subcarrier inone CDM group, a 1-OCC shift version of the OCC pattern [a b c d], thatis, [b c d a] is allocated to a DMRS subcarrier of the other CDM group,adjacent to the DRS subcarrier of the one CDM group. That is, there isan offset being a predetermined number of OCCs between the OCC patternsof two adjacent DRS subcarriers in Embodiment 7 and Embodiment 8. Thisoffset is called an OCC offset. Therefore, the OCC offset is 0, 2 and 1,respectively in Embodiment 6, Embodiment 7, and Embodiment 8. If theOCCs are allocated in the order of [a b c d] to a start DRS subcarrierof CDM group 1, the OCC patterns of CDM group 1 and CDM group 2 areformed according to Embodiment 6, Embodiment 7, and Embodiment 8 asillustrated in FIG. 29. In FIG. 29, offset-N means that there is andifference of N OCCs between the OCCs of the CDM groups. Particularly, Nis 2 in FIG. 29. In FIG. 28 illustrating Embodiment 8, the OCC patternof CDM group 2 has a left offset of 1 with respect to the OCC pattern ofCDM group 1. As illustrated in FIG. 30, the OCCs may be allocated to CDMgroup 2 with a right offset of 1, that is, a left offset of 3 withrespect to the OCC pattern of CDM group 1. If the OCC offset is 2, leftshift and right shift lead to the same result.

The OCC offset of the second CDM group with respect to the first CDMgroup may be fixed or set by a BS. It is also possible to vary the OCCoffset depending on frequency positions to more uniformly distribute theOCCs. In addition, the OCC offset may be changed according to a rankand/or a transmission mode.

An embodiment in which the OCC pattern of a DRS subcarrier of a secondCDM group adjacent to a DRS subcarrier of a first CDM group has aspecific offset with respect to the OCC pattern of the DRS subcarrier ofthe first CDM group can be implemented irrespective of how OCCs areallocated to the first CDM group. That is, while Embodiment 6,Embodiment 7, and Embodiment 8 allocate OCCs to two CDM groups with aspecific OCC offset on the assumption that OCCs are allocated to thefirst CDM group according to Embodiment 4, the same thing is alsoapplicable to Embodiment 1 to Embodiment 5.

In the embodiments in which OCCs are allocated to two CDM groups with apredetermined OCC offset, spreading OCCs and DRS ports are in thefollowing mapping relationship, for CDM group 1 and CDM group 2 whoseDRS subcarriers are adjacent. An OCC offset of 2 is given for [Table 5].

TABLE 5 Orthogonal Cover Code DRS port [w_(p)(0) w_(p)(1) w_(p)(2)w_(p)(3)] CDM group 0 [+1 +1 +1 +1] 1 1 [+1 −1 +1 −1] 1 2 [+1 +1 +1 +1]2 3 [+1 −1 +1 −1] 2 4 [+1 +1 −1 −1] 1 5 [−1 −1 +1 +1] 2 6 [+1 −1 −1 +1]1 7 [−1 +1 +1 −1] 2

DRS ports allocated to two CDM groups on two adjacent DRS subcarriersand orthogonal codes used to spread layers corresponding to the DRSports are summarized as follows.

In [Table 5], DRS port 0 to DRS port 7 may be mapped to layer 0 to layer7 in a one-to-one correspondence. In this case, the spreading OCCs ofthe DRS ports are the spreading OCCs of the layers.

TABLE 6 CDM group 1 CDM group 2 DRS Orthogonal Cover Code DRS OrthogonalCover Code port [w_(p)(0) w_(p)(1) w_(p)(2) w_(p)(3)] port [w_(p)(0)w_(p)(1) w_(p)(2) w_(p)(3)] 0 [+1 +1 +1 +1] 2 [+1 +1 +1 +1] 1 [+1 −1 +1−1] 3 [+1 −1 +1 −1] 4 [+1 +1 −1 −1] 5 [−1 −1 +1 +1] 6 [+1 −1 −1 +1] 7[−1 +1 +1 −1]

In [Table 5] and [Table 6], w_(p)(l′) is a weight multiplied with alayer in DRS OFDM symbol l′. A DRS of a DRS port is extended by aspreading OCC [w_(p)(0) w_(p)(1) w_(p)(2) w_(p)(3)] and mapped to fourDRS OFDM symbols in a subframe. A vector of weights for DRS portsallocated to a CDM group may be regarded as a multiplexing OCC. Forexample, referring to [Table 6], w_(p)(0) for DRS ports 0, 1, 4, and 6allocated to CDM group 1 is +1, +1, +1, +1. Therefore, a multiplexingOCC allocated to a start DRS OFDM symbol on a start DRS subcarrier ofCDM group 1 is a sequence a, [+1 +1 +1 +1]. w_(p)(0) for DRS ports 2, 3,5, and 7 allocated to CDM group 2 is +1, +1, −1, −1. Therefore, amultiplexing OCC allocated to a start DRS OFDM symbol on a start DRSsubcarrier of CDM group 2 is a sequence c, [+1 +1 −1 −1].

FIG. 31 illustrates OCC allocation so that there is a predetermined OCCoffset between two CDM groups according to embodiments of the presentinvention. Specifically, FIG. 31( a) illustrates OCC allocation to theother CDM group with an OCC offset of 2, when OCCs are allocated to oneCDM group according to Embodiment 1 (see FIG. 18) and FIG. 31( a)illustrates OCC allocation to the other CDM group with an OCC offset of2, when OCCs are allocated to one CDM group according to Embodiment 4(see FIG. 20( b)).

Referring to FIG. 31( a), OCCs are allocated to each DRS subcarrier ofCDM group 1 in the pattern [a b c d], starting with the OCC a. The OCCsare allocated to each DRS subcarrier of CDM group 2 with an OCC offsetof 2 with respect to the OCC pattern of CDM group 1, thus in the pattern[c d a b]. This OCC allocation scheme may be represented as thefollowing Formula.

$\begin{matrix}{{a_{k,l}^{(p)} = {{{\overset{\_}{w}}_{p}( l^{\prime} )} \cdot {r( {{3 \cdot l^{\prime} \cdot N_{RB}^{\max,{DL}}} + {3 \cdot n_{PRB}} + m^{\prime}} )}}}{where}{{{\overset{\_}{w}}_{p}(i)} = {w_{p}(i)}}{k = {{5 \cdot m^{\prime}} + {N_{sc}^{RB} \cdot n_{PRB}} + k^{\prime}}}k^{\prime} = \{ {{\begin{matrix}1 & {p \in \{ {0,1,4,6} \}} \\0 & {p \in \{ {2,3,5,7} \}}\end{matrix}l} = \{ {{\begin{matrix}{{l^{\prime}{mod}\; 2} + 2} & {{{if}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 3},4,{{or}\mspace{14mu} 8}} \\{{l^{\prime}{mod}\; 2} + 2 + {3 \cdot \lfloor {l^{\prime}/2} \rfloor}} & {{{if}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 1},2,6,{{or}\mspace{11mu} 7}} \\{{l^{\prime}{mod}\; 2} + 5} & {{if}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}}\end{matrix}l^{\prime}} = \{ {{{\begin{matrix}{0,1,2,3} & {{{{if}\mspace{14mu} n_{s}{mod}\; 2} = {0\mspace{14mu}{and}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 1}},2,6,{{or}\mspace{14mu} 7}} \\{0,1} & {{{{if}\mspace{14mu} n_{s}{mod}\; 2} = {0\mspace{14mu}{and}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 1}},2,6,{{or}\mspace{14mu} 7}} \\{2,3} & {{{{if}\mspace{14mu} n_{s}{mod}\; 2} = {1\mspace{14mu}{and}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu} 1}},2,6,{{or}\mspace{14mu} 7}}\end{matrix}m^{\prime}} = 0},1,2} } } } & \lbrack {{Formula}\mspace{14mu} 4} \rbrack\end{matrix}$where w_(p)(i) is given in [Table 5], and if k′=0, k′ represents a DRSport allocated to CDM group 1, and if k′=1, k′ represents a DRS portallocated to CDM group 2. DRS port 0 to DRS port 7 may correspond toantenna port 7 to antenna port 14 illustrated in FIGS. 10 to 16.

When the OCCs are allocated to a CDM group according to Embodiment 1, aDRS OFDM symbol contains only one OCC for each CDM group. Therefore,only two OCCs exist for the two CDM groups in the DRS OFDM symbol. Asillustrated in FIG. 31( b), the OCCs may be allocated in such a mannerthat a DRS OFDM symbol contains all OCCs in the two CDM groups.

Referring to FIG. 31( b), the OCCs are allocated in the order of [a b cd] to a DRS subcarrier of CDM group 1, starting with the OCC a, and theOCCs are allocated in the reverse order of [a b c d], that is, in theorder of [d c b a] to the next DRS subcarrier in CDM group 1. That is,the OCC allocation orders of one DRS subcarrier and the next DRSsubcarrier are reverse to each other in CDM group 1. Meanwhile, the OCCsare allocated to each DRS subcarrier of CDM group 2, with an OCC offsetof 2 with respect to the OCC pattern of a DRS subcarrier of CDM group 1adjacent to the DRS subcarrier of CDM group 2. For example, when theOCCs are allocated in the order of [a b c d] to a DRS subcarrier of CDMgroup 1, the OCCs are allocated in the order of [c d a b] to a DRSsubcarrier of CDM group 2 adjacent to the DRS subcarrier of CDM group 1.When the OCCs are allocated in the order of [d c b a] to a DRSsubcarrier of CDM group 1, the OCCs are allocated in the order of [b a dc] to a DRS subcarrier of CDM group 2 adjacent to the DRS subcarrier ofCDM group 1. Thus, the OCC patterns [a b c d] and [d c b a] alternatebetween DRS subcarriers in CDM group 1 and the OCC patterns [c d a b]and [b a d c] alternate between DRS subcarriers in CDM group 2. This OCCallocation scheme is expressed as

$\begin{matrix}{{a_{k,l}^{(p)} = {{{\overset{\_}{w}}_{p}( l^{\prime} )} \cdot {r( {{3 \cdot l^{\prime} \cdot N_{RB}^{\max,{DL}}} + {3 \cdot n_{PRB}} + m^{\prime}} )}}}{where}{{{\overset{\_}{w}}_{p}(i)} = \{ {{\begin{matrix}{w_{p}(i)} & {{( {m^{\prime} + n_{PRB}} ){mod}\; 2} = 0} \\{w_{p}( {3 - i} )} & {{( {m^{\prime} + n_{PRB}} ){mod}\; 2} = 1}\end{matrix}k} = {{{5 \cdot m^{\prime}} + {N_{sc}^{RB} \cdot n_{PRB}} + {k^{\prime}k^{\prime}}} = \{ {{\begin{matrix}1 & {p \in \{ {0,1,4,6} \}} \\0 & {p \in \{ {2,3,5,7} \}}\end{matrix}l} = \{ {{\begin{matrix}{{l^{\prime}{{mod}2}} + 2} & {{{if}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 3},4,{{or}\mspace{14mu} 8}} \\{{l^{\prime}{mod}\; 2} + 2 + {3 \cdot \lfloor {l^{\prime}/2} \rfloor}} & {{{if}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 1},2,6,{{or}\mspace{14mu} 7}} \\{{l^{\prime}{mod}\; 2} + 5} & {{if}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}}\end{matrix}l^{\prime}} = \{ {{{\begin{matrix}{0,1,2,3} & {{{{if}\mspace{14mu} n_{s}{mod}\; 2} = {0\mspace{14mu}{and}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 1}},2,6,{{or}\mspace{14mu} 7}} \\{0,1} & {{{{if}\mspace{14mu} n_{s}{mod}\; 2} = {0\mspace{14mu}{and}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}\mspace{14mu} 1}},2,6,{{or}\mspace{14mu} 7}} \\{2,3} & {{{{if}\mspace{14mu} n_{s}{mod}\; 2} = {1\mspace{14mu}{and}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu} 1}},2,6,{{or}\mspace{14mu} 7}}\end{matrix}m^{\prime}} = 0},1,2} } } }} }} & \lbrack {{Formula}\mspace{14mu} 5} \rbrack\end{matrix}$where w_(p)(i) is given in [Table 5], and if k′=0, k′ represents a DRSport allocated to CDM group 1, and if k′=1, k′ represents a DRS portallocated to CDM group 2. DRS port 0 to DRS port 7 may correspond toantenna port 7 to antenna port 14 illustrated in FIGS. 10 to 16.

When OCCs are allocated to one CDM group according to Embodiment 3-2 andthe OCCs are allocated to the other CDM group with an OCC offset of 2with respect to the one CDM group, all of the four OCCs can be used forthe two CDM groups in a DRS OFDM symbol. If OCCs are allocated with apredetermined OCC offset between CDM groups, the number of OCC pairsthat can be allocated to two adjacent DRS subcarriers of the CDM groupsis limited to 2 per DRS OFDM symbol. For example, referring to FIG. 31,given an OCC offset of 2, only OCC pairs (a, c) and (b, d) can beallocated to two adjacent DRS subcarriers of different CDM groups in aDRS OFDM symbol.

FIGS. 32 to 38 are views referred to for describing advantages ofallocating OCCs so that there is a predetermined OCC offset between twoCDM groups according to embodiments of the present invention.

It is assumed that eight DRS ports are mapped to eight layers in aone-to-one correspondence, and OCCs are allocated to two CDM groups asillustrated in FIG. 32. If the OCCs are allocated in the pattern of FIG.32 and a common scrambling sequence is applied to all layers, power maybe increased in a specific OFDM symbol, or DRS signals arecounterbalanced with one another on a DRS subcarrier of a specific OFDMsymbol, thus decreasing the power of the specific OFDM symbol.

Let a DRS port corresponding to layer m be denoted by DRS port m. Thenif multiplexing OCCs are allocated as illustrated in FIG. 32, each layermay be spread as illustrated in FIG. 33( a). In FIG. 33, s_(i)represents the position of a DRS OFDM symbol in a subframe. From theviewpoint of a layer, s_(i), s_(i+1), s_(i+2) and s_(i+3) have the samevalue. CDM #1 and CDM #2 represent CDM group 1 and CDM group 2,respectively.

Referring to FIG. 33, a DRS for each layer is spread with apredetermined spreading OCC, multiplied by a precoding matrix W in theprecoder 304, and then distributes to the RE mappers 305, correspondingrespectively to Ant #0 to Ant #7, which is expressed as

$\begin{matrix}{\begin{pmatrix}{{Ant}{\# 0}} \\{{Ant}{\# 1}} \\{{Ant}{\# 2}} \\{{Ant}{\# 3}} \\{{Ant}{\# 4}} \\{{Ant}{\# 5}} \\{{Ant}{\# 6}} \\{{Ant}{\# 7}}\end{pmatrix} = {{W \times \begin{pmatrix}{{layer}\; 0 \times \begin{bmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1}\end{bmatrix}} \\{{layer}\; 1 \times \begin{bmatrix}{+ 1} & {- 1} & {+ 1} & {- 1}\end{bmatrix}} \\{{layer}\; 2 \times \begin{bmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1}\end{bmatrix}} \\{{layer}\; 3 \times \begin{bmatrix}{+ 1} & {- 1} & {+ 1} & {- 1}\end{bmatrix}} \\{{layer}\; 4 \times \begin{bmatrix}{+ 1} & {+ 1} & {- 1} & {- 1}\end{bmatrix}} \\{{layer}\; 5 \times \begin{bmatrix}{+ 1} & {+ 1} & {- 1} & {- 1}\end{bmatrix}} \\{{layer}\; 6 \times \begin{bmatrix}{+ 1} & {- 1} & {- 1} & {+ 1}\end{bmatrix}} \\{{layer}\; 7 \times \begin{bmatrix}{+ 1} & {- 1} & {- 1} & {+ 1}\end{bmatrix}}\end{pmatrix}} = {\begin{pmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {+ j} & {- 1} & {- j} & {+ 1} & {+ j} & {- 1} & {- j} \\{+ 1} & {- 1} & {+ 1} & {- 1} & {+ 1} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {- j} & {- 1} & {+ j} & {+ 1} & {- j} & {- 1} & {+ j} \\{+ 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} & {- 1} & {- 1} & {- 1} \\{+ 1} & {+ j} & {- 1} & {- j} & {- 1} & {- j} & {+ 1} & {+ j} \\{+ 1} & {- 1} & {+ 1} & {- 1} & {- 1} & {+ 1} & {- 1} & {+ 1} \\{+ 1} & {- j} & {- 1} & {+ j} & {- 1} & {+ j} & {+ 1} & {- j}\end{pmatrix} \times \begin{pmatrix}s_{i} & s_{i + 1} & s_{i + 2} & s_{i + 3} \\s_{i} & {- s_{i + 1}} & s_{i + 2} & {- s_{i + 3}} \\s_{i} & s_{i + 1} & s_{i + 2} & s_{i + 3} \\s_{i} & {- s_{i + 1}} & s_{i + 2} & {- s_{i + 3}} \\s_{i} & s_{i + 1} & {- s_{i + 2}} & {- s_{i + 3}} \\s_{i} & s_{i + 1} & {- s_{i + 2}} & {- s_{i + 3}} \\s_{i} & {- s_{i + 1}} & {- s_{i + 2}} & s_{i + 3} \\s_{i} & {- s_{i + 1}} & {- s_{i + 2}} & s_{i + 3}\end{pmatrix}}}} & \lbrack {{Formula}\mspace{14mu} 6} \rbrack\end{matrix}$

Referring to FIG. 33( b), Ant #0 may require very high power for DRSOFDM symbol 0 and Ant #4 may require very high power for DRS OFDM symbol2. For two PRBs in a subframe, the power ratios among OFDM symbolsallocated to Ant #0 are calculated as illustrated in FIG. 34. Given apower of 1 for a data RE, a power per RE in each OFDM symbol iscalculated over two PRBs in FIG. 34. Referring to FIG. 34, for Ant #0,the first DRS OFDM symbol has a peak power, while no power is allocatedto the other DRS OFDM symbols. Thus the other DRS OFDM symbols are at alower power level than non-DRS OFDM symbols.

Meanwhile, if OCCs are allocated with a predetermined offset between twoCDM groups according to the present invention, for example, if OCCs areallocated as illustrated in FIG. 31( a), DRSs are distributed to Ant #0to Ant #7 as follows.

$\begin{matrix}{\begin{pmatrix}{{Ant}{\# 0}} \\{{Ant}{\# 1}} \\{{Ant}{\# 2}} \\{{Ant}{\# 3}} \\{{Ant}{\# 4}} \\{{Ant}{\# 5}} \\{{Ant}{\# 6}} \\{{Ant}{\# 7}}\end{pmatrix} = {{W \times \begin{pmatrix}{{layer}\; 0 \times \begin{bmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1}\end{bmatrix}} \\{{layer}\; 1 \times \begin{bmatrix}{+ 1} & {- 1} & {+ 1} & {- 1}\end{bmatrix}} \\{{layer}\; 2 \times \begin{bmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1}\end{bmatrix}} \\{{layer}\; 3 \times \begin{bmatrix}{+ 1} & {- 1} & {+ 1} & {- 1}\end{bmatrix}} \\{{layer}\; 4 \times \begin{bmatrix}{+ 1} & {+ 1} & {- 1} & {- 1}\end{bmatrix}} \\{{layer}\; 5 \times \begin{bmatrix}{- 1} & {- 1} & {+ 1} & {+ 1}\end{bmatrix}} \\{{layer}\; 6 \times \begin{bmatrix}{+ 1} & {- 1} & {- 1} & {+ 1}\end{bmatrix}} \\{{layer}\; 7 \times \begin{bmatrix}{- 1} & {+ 1} & {+ 1} & {- 1}\end{bmatrix}}\end{pmatrix}} = {\begin{pmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {+ j} & {- 1} & {- j} & {+ 1} & {+ j} & {- 1} & {- j} \\{+ 1} & {- 1} & {+ 1} & {- 1} & {+ 1} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {- j} & {- 1} & {+ j} & {+ 1} & {- j} & {- 1} & {+ j} \\{+ 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} & {- 1} & {- 1} & {- 1} \\{+ 1} & {+ j} & {- 1} & {- j} & {- 1} & {- j} & {+ 1} & {+ j} \\{+ 1} & {- 1} & {+ 1} & {- 1} & {- 1} & {+ 1} & {- 1} & {+ 1} \\{+ 1} & {- j} & {- 1} & {+ j} & {- 1} & {+ j} & {+ 1} & {- j}\end{pmatrix} \times \begin{pmatrix}s_{i} & s_{i + 1} & s_{i + 2} & s_{i + 3} \\s_{i} & {- s_{i + 1}} & s_{i + 2} & {- s_{i + 3}} \\s_{i} & s_{i + 1} & s_{i + 2} & s_{i + 3} \\s_{i} & {- s_{i + 1}} & s_{i + 2} & {- s_{i + 3}} \\s_{i} & s_{i + 1} & {- s_{i + 2}} & {- s_{i + 3}} \\{- s_{i}} & {- s_{i + 1}} & s_{i + 2} & s_{i + 3} \\s_{i} & {- s_{i + 1}} & {- s_{i + 2}} & s_{i + 3} \\{- s_{i}} & {+ s_{i + 1}} & {+ s_{i + 2}} & {- s_{i + 3}}\end{pmatrix}}}} & \lbrack {{Formula}\mspace{14mu} 7} \rbrack\end{matrix}$

Among antenna-specific symbols distributed according to [Formula 7], thepower of symbols distributed to Ant #0 may be represented as illustratedin FIG. 35. Compared to FIG. 34 where the OFDM symbol power ranges from−3.1 dB to 3.98 dB, it ranges from −3.1 dB to 2.2 dB in FIG. 35( a) andthus has a reduced power variation. The power of symbols allocated toAnt #0 over one RB of a subframe may be represented as illustrated inFIG. 36.

Meanwhile, if OCCs are allocated as illustrated in FIG. 31( b), powermay be more uniformly distributed across OFDM symbols as illustrated inFIG. 36.

Embodiment 9

Phase offsets may be used to eliminate power imbalance. Embodiment 9seeks power balancing by applying a phase offset to at least one of CDMgroups. Embodiment 9 may be implemented in combination with any of theforegoing embodiments for eliminating power imbalance. It is alsopossible to use only a phase offset without an OCC offset.

FIGS. 37 and 38 illustrate OCC allocation using phase offsets accordingto an embodiment of the present invention.

In FIG. 37, two phase offsets are alternately applied only to CDM group2, with no OCC offset, that is, with an OCC offset of 0. Referring toFIG. 37( a), DRSs multiplexed to CDM group 2 are alternately multipliedby two phase offsets θ_(a) and θ_(b) on DRS subcarriers.

In FIG. 38, two phase offsets are alternately applied to CDM group 2,with an OCC offset of 2. Specifically, OCCs are allocated as illustratedin FIG. 31( b) and DRSs multiplexed in CDM group 2 are multipliedalternately by two phase offsets θ_(a) and θ_(b) on DRS sub carriers.

For instance, if θ_(a) and θ_(b) are 0 and π, respectively, the DRSsmultiplexed in CDM group 2 are multiplied by 1 and −1, alternately onDRS subcarriers, as illustrated in FIGS. 37( b) and 38(b).

Embodiment 10

Different phases may be applied according to DRS ports. While the samephase offset is applied to each DRS subcarrier for all DRS portsallocated to a CDM group in Embodiment 9, different phase offsets areapplied according to DRS ports in Embodiment 10. That is, differentphase offsets are multiplied with layers on the same DRS subcarrier. Inaddition, the same phase offset is applied to layers spread with thesame OCC and DRSs corresponding to the layers. To repeat the same OCCpattern in every predetermined number of RBs, a phase offset may be setsuch that the product of the phase offset and the number of DRSsubcarriers included in the predetermined number of RBs is an integermultiple of 2π.

FIG. 39 illustrates phase offsets applied to DRS subcarriers for eachDRS port. Particularly, FIG. 39 illustrates a case where the layer andDRS of each DRS port are spread with a spreading OCC listed in [Table3]. In FIG. 39, subcarriers 0, 5 and 10 are the logical indexes ofsubcarriers in an RB, mapped to DRS subcarriers 0, 1 and 2.

Referring to FIG. 39, irrespective of CDM groups, phase offsets areapplied in the same pattern to layers and DRSs corresponding to DRS port0 and DRS port 2, to layers and DRSs corresponding to DRS port 1 and DRSport 3, to layers and DRSs corresponding to DRS port 4 and DRS port 5,and to layers and DRSs corresponding to DRS port 6 and DRS port 7.Referring to FIG. 39( a), DRS subcarriers have a phase offset of 0 foreach DRS port. Referring to FIG. 39( b), DRS subcarriers have a phaseoffset of π for each DRS port. In FIGS. 39( c) and 39(d), ω ise^(j(π/3)). The phase offset between DRS subcarriers is π/3 for each DRSport in FIG. 39( c) and −π/3 for each DRS port in FIG. 39( d).

FIGS. 40, 41 and 42 are views referred to for describing advantages ofapplying phase offsets according to DRS subcarriers for each layeraccording to embodiments of the present invention.

FIG. 40 illustrates DRSs distributed to Ant #0 over two RBs in asubframe, when a phase offset is applied on a layer basis without an OCCoffset between CDM groups. In FIG. 40, phase offsets are appliedaccording to DRS subcarriers for each layer as illustrated in FIG. 39and the precoding matrix illustrated in FIG. 34 is used.

Referring to FIG. 41, when phase offsets are applied according to DRSsubcarriers for each layer, a uniform power distribution across OFDMsymbols in two RBs can be achieved. According to this embodiment,however, since different phase offsets are applied to different layersas well as different DRS subcarriers, multiplexing of a plurality oflayers is complicated. Compared to other embodiments related to powerbalancing based on an OCC offset between CDM groups, this embodimentrequires higher-performance processors 400 a and 400 b for a transmitterand a receiver.

Power is uniformly distributed across OFDM symbols in an even number ofRBs as illustrated in FIG. 41, whereas power imbalance still exists inan odd number of RBs as illustrated in FIG. 42. Perfect power balancemay not be achieved for an odd number of RBs, only with phase offsets.

FIG. 43 is a view referred to for describing advantages achieved whenOCCs are allocated so that there is a predetermined OCC offset betweentwo CDM groups and phase offsets are applied according to DRSsubcarriers for each layer according to embodiments of the presentinvention.

As noted from FIG. 43, the use of both an OCC offset and a phase offsetmay lead to a more uniform power distribution even for an odd number ofRBs.

According to the present invention, a BS may spread a DRS for each layerwith a predetermined spreading OCC in one of the afore-describedembodiments. The BS precodes the spread DRSs with a predeterminedprecoding matrix, thus outputting antenna-specific symbols. For example,referring to FIG. 33, the BS spreads a part or all of layer 0 to layer 8with predetermined Walsh codes and precodes the spread layers with aprecoding matrix W, thus distributing the precoded symbols to a part orall of Ant #0 to Ant #7. The distributed symbols are converted to anOFDM signal and transmitted to a UE(s) within the coverage of the BS.

According to the present invention, the BS processor 400 b may allocateone or more layers to a specific subframe. In this case, the BSprocessor 400 b may allocate DRSs for demodulation of the respectivelayers to the specific subframe. The BS transmitter 100 b transmits theallocated layers along with the DRSs under the control of the BSprocessor 400 b.

The BS processor 400 b may control the BS transmitter 100 b to transmitthe DRSs in one or more CDM groups according to one of theafore-described embodiments. For this purpose, the BS processor 400 bmay allocate spreading OCCs to the layers according to one of theafore-described embodiments. The BS processor 400 b spreads a DRS (DRSs)corresponding to a transmission layer(s) with the predeterminedspreading code(s) and controls the BS transmitter 100 b to allocate thespread DRSs to a predetermined CDM group. The BS transmitter 100 b maytransmit the spread DRSs in the CDM group under the control of the BSprocessor 400 b. The RE mappers 305 map the elements of the spread DRSsequences to DRS REs of the CDM group under the control of the BSprocessor 400 b.

That is, the BS processor 400 b may allocate a multiplexing OCC to oneor more CDM groups according to one of the afore-described embodiments.The BS processor 400 b multiplexes a plurality of DRSs using amultiplexing OCC allocated to a DRS RE. The BS transmitter 400 btransmits the multiplexed DRSs on the DRS RE.

Under the control of the BS processor 400 b, the BS transmitter 100 bspreads a DRS for each layer, maps each element of the spread DRS to oneDRS RE, and transmit the DRS on the mapped DRS RE(s). The RE mappers 305map a layer(s) and a DRS(s) corresponding to the layer(s) to a subframe.The OFDM/SC-FDM signal generators 306 convert the mapped layer(s) andDRS(s) to an OFDM signal and the OFDM signal is transmitted to a UE(s)within the coverage of the BS.

A UE receives the OFDM signal from the BS and recovers antenna-specificsymbols from the received OFDM signal. The UE recovers one or more layersignals from the antenna-specific symbols using the precoding matrix Wused in the BS. The precoding matrix W may be preset between the UE andthe BS. Alternatively or additionally, the UE or BS may select anappropriate precoding matrix W and signal it to the BS or UE.

The UE may detect a layer and/or a DRS destined for the UE from amongthe recovered layer signals. For example, referring to FIG. 33, the UEmay recover signals of DRS REs by recovering antenna-specific symbolsfrom the received OFDM signals as illustrated in FIG. 33( b). The UErecovers one or more layer signals using the precoding matrix W from theDRS RE signals. If the BS transmits a plurality of layers, a pluralityof DRSs are multiplexed in DRS REs. The UE may acquire a valuecorresponding to an integer multiple of the layer signals by multiplyingthe spreading OCCs used for spreading the layers for the UE by themultiplexed signal.

For example, referring to FIG. 33( a), it is assumed that the UEreceives the spread DRSs of layer 0, layer 1, layer 4 and layer 6 on aDRS subcarrier of CDM group 1 (CDM #1) over four DRS OFDM symbols. Let areference signal for layer i be denoted by DRS i. Then the signal thatthe UE has received on the DRS subcarrier of CDM group 1 over four DRSOFDM symbols may be related to (DRS 0)×[+1 +1 +1 +1]+(DRS 1)×[+1 −1 +1−1]+(DRS 4)×[+1 +1 −1 −1]+(DRS 6)×[+1 −1 −1 +1]. If layer 1 is destinedfor the UE, the UE may extract DRS 1 by multiplying the received signalby the spreading OCC used for layer 1, [+1 −1 +1 −1]^(T). The UE maydemodulate the layer using the DRS of the layer.

According to the present invention, the UE receiver 300 a may receiveone or more layers from the BS. The UE receiver 300 a may receive fromthe BS one or more DRSs multiplexed in one or more CDM groups, destinedfor the UE according to one of the foregoing embodiments of the presentinvention. The UE processor 400 a controls the UE receiver 300 a toconvert the received OFDM signal to a baseband signal. The UE receiver300 a generates antenna-specific symbols by demapping the basebandsignal from REs under the control of the UE processor 400 a. Under thecontrol of the UE processor 400 a, the UE receiver 300 a recovers theone or more layers transmitted by the BS from the antenna-specificsymbols using the precoding matrix used for precoding by the BS. Todemodulate a layer destined for the UE from among the one or morelayers, the UE receiver 300 a detects a DRS of the layer using aspreading OCC corresponding to the layer under the control of the UEprocessor 400 a. The spreading OCC used for detection of the layer isdetermined according to an afore-described embodiment of the presentinvention. The UE processor 400 a may control the UE receiver 300 a todemodulate the layer using the detected DRS.

While the above embodiments of the present invention has been describedin the context that OCCs of length 4 are multiplexed in two CDM groups,they are also applicable to multiplexing of OCCs of any other lengthinto any other number of CDM groups. For instance, power balance acrossOFDM symbols can be achieved by implementing the embodiments of thepresent invention in the same manner, when OCCs of a length larger than4 are multiplexed in one or two CDM groups or in three or more CDMgroups.

As is apparent from the above description, the present inventionuniformly distributes transmission power across all OFDM symbols of asubframe.

The embodiments of the present invention can be applied to a BS, a UE,or other communication devices in a wireless communication system.

It will be apparent to those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

What is claimed is:
 1. A method for transmitting a plurality ofreference signals (RSs) from a base station (BS) to a user equipment(UE) in a wireless communication system, the method comprising:multiplexing, by the BS, the plurality of RSs in each of a first codedivision multiplexing (CDM) group and a second CDM group; andtransmitting the multiplexed plurality of RSs by the BS to the UE,wherein the multiplexing comprises: multiplexing RS0, RS1, RS4 and RS 6among the plurality of RSs on a subcarrier of the first CDM group,hereinafter referred to as a first subcarrier, according to (RS0 RS1 RS4RS6)×(a b c d), and multiplexing RS2, RS3, RS5 and RS7 among theplurality of RSs on a subcarrier of the second CDM group, hereinafterreferred to as a second subcarrier, which is adjacent to the firstsubcarrier, according to (RS2 RS3 RS5 RS7)×(c d a b), and wherein the a,b ,c and d are orthogonal sequences defined as $\begin{matrix}{W_{4} = \begin{pmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {+ 1} & {- 1} & {- 1} \\{+ 1} & {- 1} & {- 1} & {+ 1}\end{pmatrix}} \\{= {\begin{pmatrix}a & b & c & d\end{pmatrix}.}}\end{matrix}$
 2. The method according to claim 1, wherein the RS0 to RS7are mapped to 8 antenna ports of a base station (BS), respectively. 3.The method according to claim 2, wherein the RS0 to RS7 are multiplexedon the first and second subcarriers in an orthogonal frequency divisionmultiplexing (OFDM) symbol by using one of multiplexing orthogonalsequence pairs (a, c) and (b, d).
 4. The method according to claim 1,wherein the plurality of RSs are spread and multiplexed according to afollowing table: Orthogonal sequence w_(p)(i) RSp [w_(p)(0) w_(p)(1)w_(p)(2) w_(p)(3)] CDM group 0 [+1 +1 +1 +1] 1 1 [+1 −1 +1 −1] 1 2 [+1+1 +1 +1] 2 3 [+1 −1 +1 −1] 2 4 [+1 +1 −1 −1] 1 5 [−1 −1 +1 +1] 2 6 [+1−1 −1 +1] 1 7 [−1 +1 +1 −1]  2,

where CDM group 1 and CDM group 2 is the first CDM group and the secondCDM group, respectively, and w_(p)(i)(i=0,1,2,3) is a weight multipliedwith RSp in an i-th OFDM symbol among 4 OFDM symbols used fortransmission of the plurality of RSs in a subframe.
 5. A method forreceiving a plurality of reference signals (RSs) at a user equipment(UE) from a base station (BS) in a wireless communication system, themethod comprising: receiving, by the UE from the BS, the plurality ofRSs, the received plurality of RSs having been multiplexed by the BS ineach of a first code division multiplexing (CDM) group and a second CDMgroup, wherein RS0, RS1, RS4 and RS 6 among the plurality of RSs aremultiplexed on a subcarrier of the first CDM group, hereinafter referredto as a first subcarrier, according to (RS0 RS1 RS4 RS6)×(a b c d), andRS2, RS3, RS5 and RS7 among the plurality of RSs are multiplexed on asubcarrier of the second CDM group, hereinafter referred to as a secondsubcarrier, which is adjacent to the first subcarrier, according to (RS2RS3 RS5 RS7)×(c d a b), and wherein the a, b, c and d are orthogonalsequences defined as $\begin{matrix}{W_{4} = \begin{pmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {+ 1} & {- 1} & {- 1} \\{+ 1} & {- 1} & {- 1} & {+ 1}\end{pmatrix}} \\{= {\begin{pmatrix}a & b & c & d\end{pmatrix}.}}\end{matrix}$
 6. The method according to claim 5, wherein the RS0 to RS7are mapped to 8 antenna ports of a base station (BS), respectively. 7.The method according to claim 6, wherein the RS0 to RS7 are multiplexedon the first and second subcarriers in an orthogonal frequency divisionMultiplexing (OFDM) symbol by using one of multiplexing orthogonalsequence pairs (a, c) and (b, d).
 8. The method according to claim 5,wherein the plurality of RSs are spread and multiplexed according to afollowing table: Orthogonal sequence w_(p)(i) RSp [w_(p)(0) w_(p)(1)w_(p)(2) w_(p)(3)] CDM group 0 [+1 +1 +1 +1] 1 1 [+1 −1 +1 −1] 1 2 [+1+1 +1 +1] 2 3 [+1 −1 +1 −1] 2 4 [+1 +1 −1 −1] 1 5 [−1 −1 +1 +1] 2 6 [+1−1 −1 +1] 1 7 [−1 +1 +1 −1]  2,

where CDM group 1 and CDM group 2 is the first CDM group and the secondCDM group, respectively, and w_(p)(i) (i=0,1,2,3) is a weight multipliedwith RSp in an i-th OFDM symbol among 4 OFDM symbols used fortransmission of the plurality of RSs in a subframe.
 9. A base station(BS) for transmitting a plurality of reference signals (RSs) to a userequipment (UE) in a wireless communication system, the BS comprising: atransmitter; and a processor, operatively coupled to the transmitter,configured to control the transmitter, wherein the processor isconfigured to multiplex the plurality of RSs with spreading orthogonalsequences on each of a first code division multiplexing (CDM) group anda second CDM group and control the transmitter to transmit themultiplexed plurality of RSs to the UE, wherein RS0, RS1, RS4 and RS 6among the plurality of RSs are multiplexed on a subcarrier of the firstCDM group, hereinafter referred to as a first subcarrier, according to(RS0 RS1 RS4 RS6)×(a b c d), and RS2, RS3, RS5 and RS7 among theplurality of RSs are multiplexed on a subcarrier of the second CDMgroup, hereinafter referred to as a second subcarrier, which is adjacentto the first subcarrier, according to (RS2 RS3 RS5 RS7)×(c d a b), andwherein the a, b, c and d are orthogonal sequences defined as$\begin{matrix}{W_{4} = \begin{pmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {+ 1} & {- 1} & {- 1} \\{+ 1} & {- 1} & {- 1} & {+ 1}\end{pmatrix}} \\{= {\begin{pmatrix}a & b & c & d\end{pmatrix}.}}\end{matrix}$
 10. The BS according to claim 9, wherein the RS0 to RS7are mapped to 8antenna ports of the BS, respectively.
 11. The BSaccording to claim 10, wherein the RS0 to RS7 are multiplexed on thefirst and second subcarriers in an orthogonal frequency divisionmultiplexing (OFDM) symbol by using one of multiplexing orthogonalsequence pairs (a, c) and (b, d).
 12. The BS according to claim 9,wherein the processor is configured to spread and multiplex theplurality of RSs according to a following table: Orthogonal sequencew_(p)(i) RSp [w_(p)(0) w_(p)(1) w_(p)(2) w_(p)(3)] CDM group 0 [+1 +1 +1+1] 1 1 [+1 −1 +1 −1] 1 2 [+1 +1 +1 +1] 2 3 [+1 −1 +1 −1] 2 4 [+1 +1 −1−1] 1 5 [−1 −1 +1 +1] 2 6 [+1 −1 −1 +1] 1 7 [−1 +1 +1 −1]  2,

where CDM group 1 and CDM group 2 is the first CDM group and the secondCDM group, respectively, and w_(p)(i) (i=0,1,2,3) is a weight multipliedwith RSp in an i-th OFDM symbol among 4 OFDM symbols used fortransmission of the plurality of RSs in a subframe.
 13. A user equipment(UE) for receiving a plurality of reference signals (RSs) from a basestation (BS) in a wireless communication system, the UE comprising: areceiver; and a processor, operatively coupled to the receiver,configured to control the receiver, wherein the processor is configuredto control the receiver to receive the plurality of RSs from the BS, thereceived plurality of RSs having been multiplexed by the BS in each afirst code division multiplexing (CDM) group and a second CDM group,wherein RS0, RS1, RS4 and RS 6 among the plurality of RSs aremultiplexed on a subcarrier of the first CDM group, hereinafter referredto as a first subcarrier, according to (RS0 RS1 RS4 RS6)×(a b c d), andRS2, RS3, RS5 and RS7 among the plurality of RSs are multiplexed on asubcarrier of the second CDM group, hereinafter referred to as a secondsubcarrier, which is adjacent to the first subcarrier, according to (RS2RS3 RS5 RS7)×(c d a b), wherein the a, b, c and d are orthogonalsequences defined as $\begin{matrix}{W_{4} = \begin{pmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {+ 1} & {- 1} & {- 1} \\{+ 1} & {- 1} & {- 1} & {+ 1}\end{pmatrix}} \\{= {\begin{pmatrix}a & b & c & d\end{pmatrix}.}}\end{matrix}$
 14. The UE according to claim 13, wherein the RS0 to RS7are mapped to 8 antenna ports of a base station (BS), respectively. 15.The UE according to claim 14, wherein the RS0 to RS7 are multiplexed onthe first and second subcarriers in an orthogonal frequency divisionmultiplexing (OFDM) symbol by using one of multiplexing orthogonalsequence pairs (a, c) and (b, d).
 16. The UE according to claim 13,wherein the processor is configured to control the receiver to receivethe plurality of RSs spread and multiplexed according to a followingtable: Orthogonal sequence w_(p)(i) RSp [w_(p)(0) w_(p)(1) w_(p)(2)w_(p)(3)] CDM group 0 [+1 +1 +1 +1] 1 1 [+1 −1 +1 −1] 1 2 [+1 +1 +1 +1]2 3 [+1 −1 +1 −1] 2 4 [+1 +1 −1 −1] 1 5 [−1 −1 +1 +1] 2 6 [+1 −1 −1 +1]1 7 [−1 +1 +1 −1]  2,

where CDM group 1 and CDM group 2 is the first CDM group and the secondCDM group, respectively, and w_(p)(i) (i=0,1,2,3) is a weight multipliedwith RSp in an i-th OFDM symbol among 4 OFDM symbols used fortransmission of the plurality of RSs in a subframe.